Resolve RTS-timing problem

On some devices there is a timing problem with the RTS line, when not using flush.
flush does wait until all data is send: https://www.arduino.cc/reference/en/language/functions/communication/serial/flush/

tasmota/xnrg_12_solaxX1.ino

@@ -162,6 +162,7 @@ void solaxX1_RS485Send(uint16_t msgLen)
   solaxX1Serial->write(message, msgLen);
   solaxX1Serial->write(highByte(crc));
   solaxX1Serial->write(lowByte(crc));
+  solaxX1Serial->flush();
   if (PinUsed(GPIO_SOLAXX1_RTS)) {
     digitalWrite(Pin(GPIO_SOLAXX1_RTS), LOW);
   }