rust/mqtt: fix clippy 1.77 warning

error: creating a mutable reference to mutable static is discouraged
   --> src/mqtt/mqtt.rs:752:23
    |
752 |     let max_msg_len = &mut MAX_MSG_LEN;
    |                       ^^^^^^^^^^^^^^^^ mutable reference to mutable static
    |
    = note: for more information, see issue #114447 <rust-lang/rust#114447>
    = note: this will be a hard error in the 2024 edition
    = note: this mutable reference has lifetime `'static`, but if the static gets accessed (read or written) by any other means, or any other reference is created, then any further use of this mutable reference is Undefined Behavior

(cherry picked from commit 80d2c6e)

rust/src/mqtt/mqtt.rs

@@ -838,8 +838,7 @@ export_tx_data_get!(rs_mqtt_get_tx_data, MQTTTransaction);
 #[no_mangle]
 pub unsafe extern "C" fn rs_mqtt_register_parser(cfg_max_msg_len: u32) {
     let default_port = CString::new("[1883]").unwrap();
-    let max_msg_len = &mut MAX_MSG_LEN;
-    *max_msg_len = cfg_max_msg_len;
+    MAX_MSG_LEN = cfg_max_msg_len;
     let parser = RustParser {
         name: PARSER_NAME.as_ptr() as *const std::os::raw::c_char,
         default_port: default_port.as_ptr(),