Fix handling of USB devices with numeric serial strings

[enkiusz]

I have a FT232RL device which has the string '4' as the USB serial number. This confuses usbtools.py as the code first tries to parse the locator as a devidx instead of a serial. The PR modifies this code path so that it checks if the locator is a sernum first.

➜  ~ sudo lsusb -d 0403:6001 -vv

Bus 003 Device 033: ID 0403:6001 Future Technology Devices International, Ltd FT232 Serial (UART) IC
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass            0 
  bDeviceSubClass         0 
  bDeviceProtocol         0 
  bMaxPacketSize0         8
  idVendor           0x0403 Future Technology Devices International, Ltd
  idProduct          0x6001 FT232 Serial (UART) IC
  bcdDevice            6.00
  iManufacturer           1 SIBIT
  iProduct                2 SIBIT USB<>RS232 TTL CONVERTER
  iSerial                 3 4
  bNumConfigurations      1
  Configuration Descriptor:
    bLength                 9
    bDescriptorType         2
    wTotalLength       0x0020
    bNumInterfaces          1
    bConfigurationValue     1
    iConfiguration          0 
    bmAttributes         0xa0
      (Bus Powered)
      Remote Wakeup
    MaxPower              100mA
    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber        0
      bAlternateSetting       0
      bNumEndpoints           2
      bInterfaceClass       255 Vendor Specific Class
      bInterfaceSubClass    255 Vendor Specific Subclass
      bInterfaceProtocol    255 Vendor Specific Protocol
      iInterface              2 SIBIT USB<>RS232 TTL CONVERTER
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x81  EP 1 IN
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               0
      Endpoint Descriptor:
        bLength                 7
        bDescriptorType         5
        bEndpointAddress     0x02  EP 2 OUT
        bmAttributes            2
          Transfer Type            Bulk
          Synch Type               None
          Usage Type               Data
        wMaxPacketSize     0x0040  1x 64 bytes
        bInterval               0
can't get device qualifier: Resource temporarily unavailable
can't get debug descriptor: Resource temporarily unavailable
Device Status:     0x0000
  (Bus Powered)


➜  ~ ftdi_urls.py 
Available interfaces:
  ftdi://ftdi:232:4/1   (SIBIT USB<>RS232 TTL CONVERTER)

➜  ~ ftconf.py ftdi://ftdi:232:4/1 -v
Traceback (most recent call last):
  File "/home/enki/.local/bin/ftconf.py", line 193, in <module>
    main()
  File "/home/enki/.local/bin/ftconf.py", line 125, in main
    eeprom.open(args.device, size=args.size, model=args.eeprom)
  File "/home/enki/.local/lib/python3.10/site-packages/pyftdi/eeprom.py", line 150, in open
    self._ftdi.open_from_url(device)
  File "/home/enki/.local/lib/python3.10/site-packages/pyftdi/ftdi.py", line 493, in open_from_url
    devdesc, interface = self.get_identifiers(url)
  File "/home/enki/.local/lib/python3.10/site-packages/pyftdi/ftdi.py", line 395, in get_identifiers
    return UsbTools.parse_url(url,
  File "/home/enki/.local/lib/python3.10/site-packages/pyftdi/usbtools.py", line 328, in parse_url
    raise UsbToolsError('No USB device matches URL %s' %
pyftdi.usbtools.UsbToolsError: No USB device matches URL ftdi://ftdi:232:4/1
➜  ~ 

After:

➜  pyftdi git:(master) ✗ ftdi_urls.py 
Available interfaces:
  ftdi://ftdi:232:4/1   (SIBIT USB<>RS232 TTL CONVERTER)

➜  pyftdi git:(master) ✗ ftconf.py ftdi://ftdi:232:4/1 -v
vendor_id: 0x0403
product_id: 0x6001
type: 0x0600
self_powered: False
remote_wakeup: True
power_max: 100
has_serial: True
suspend_pull_down: True
out_isochronous: False
in_isochronous: False
manufacturer: SIBIT
product: SIBIT USB<>RS232 TTL CONVERTER
serial: 4
channel_a_driver: VCP
high_current: True
external_oscillator: 0
invert_TXD: False
invert_RXD: False
invert_RTS: False
invert_CTS: False
invert_DTR: False
invert_DSR: False
invert_DCD: False
invert_RI: False
cbus_func_0: TXLED
cbus_func_1: RXLED
cbus_func_2: GPIO
cbus_func_3: GPIO
cbus_func_4: SLEEP