Fix repeated Uart Tx packet in Rx Uart channel

This fix allow to solve the problem of the repeated Tx packet in uart rx channel #2, it was tested with the example ModbusRTUTemperatureSensor, it show that the master unit receive only one time te packet, and that the packet is not replicated on the Uart rx channel.

Author: Rocketct

src/libmodbus/modbus-rtu.cpp

@@ -327,9 +327,11 @@ static ssize_t _modbus_rtu_send(modbus_t *ctx, const uint8_t *req, int req_lengt
 
     ssize_t size;
 
+    RS485.noReceive();
     RS485.beginTransmission();
     size = RS485.write(req, req_length);
     RS485.endTransmission();
+    RS485.receive();
 
     return size;
 #else