FrSky SmartPort bugfix

There was byte stuffing missing which could cause sporadic ommision of certain telemetry values (when they contained critical bytes).
Don't sure I understand. Rx and tx are the same context / thread in scope of this module. When frsky receiver decides to start transmitt during my transmission, it will be hw-level collision (with dominant 1 and weak 0) and I can't see how to prevent it.