AP_FETtecOneWire: reset buf_used

if byte is missing due to electrical noise it can now resync

libraries/AP_FETtecOneWire/AP_FETtecOneWire.cpp

@@ -606,6 +606,7 @@ void AP_FETtecOneWire::escs_set_values(const uint16_t* motor_values)
     // No command was yet sent, so no reply is expected and all information
     // on the receive buffer is either garbage or noise. Discard it
     _uart->discard_input();
+    _receive_buf_used = 0;
 
     // send throttle commands to all configured ESCs in a single packet transfer
     if (transmit(fast_throttle_command, sizeof(fast_throttle_command))) {