AP_FETtecOneWire: Do not write to the UART buffer if the previous transfer did not complete yet

Required for stable operation on F4 processors

libraries/AP_FETtecOneWire/AP_FETtecOneWire.cpp

@@ -798,6 +798,15 @@ void AP_FETtecOneWire::update()
         return;
     }
 
+#if defined(STM32F4)
+    if (_uart->tx_pending()) {
+        // there is unsent data in the send buffer,
+        // do not send more data because FETtec needs a time gap between frames
+        _period_too_short++;
+        return;
+    }
+#endif
+
 #if HAL_AP_FETTEC_CONFIGURE_ESCS
     // run ESC configuration state machines if needed
     if (_running_mask != _motor_mask) {