FrSky SmartPort bugfix

There was byte stuffing missing which could cause sporadic ommision of certain telemetry values (when they contained critical bytes).
Shouldn't this really be += cnt? Also looking at the other code, it would appear this _should_ be blocked by a semaphore to protect between the tx and rx code. However, if it works and is robust to frame shifts it's probably reasonable to let one or two frames potentially be missed on the RX side. It might also be the hardware timings of the smart port make this safe too.