feat(radio): Discover SBUS trainer availability (#4266)

Co-authored-by: raphaelcoeffic <1050031+raphaelcoeffic@users.noreply.github.com>
Co-authored-by: Peter Feerick <peter.feerick@gmail.com>

radio/src/serial.cpp

@@ -217,12 +217,10 @@ static void serialSetCallBacks(int mode, void* ctx, const etx_serial_port_t* por
     break;
 #endif
 
-#if defined(SBUS_TRAINER)
   case UART_MODE_SBUS_TRAINER:
     sbusSetAuxGetByte(ctx, getByte);
     // TODO: setRxCb (see MODE_LUA)
     break;
-#endif
 
   case UART_MODE_TELEMETRY:
     // telemetrySetGetByte(ctx, getByte);