Add serial port baudrate setter

Allow serial port code to set the baud rate as well as query. Avoid
re-setting the baud rate to the current value as QtSerial seems to
do two baud rate settings, first to 4Mbaud and then to the target rate
in this case.

Signed-off-by: Keith Packard <keithp@keithp.com>

mu/modes/base.py

@@ -106,6 +106,13 @@ def baudrate(self):
         else:
             return None
 
+    @baudrate.setter
+    def baudrate(self, value):
+        if value != self._baudrate:
+            self._baudrate = value
+            if self.serial:
+                self.serial.setBaudRate(value)
+
     def open(self):
         """
         Open the serial link