removing avr register dependency

trying to port existing code to ESP8266.
It does not has UCSR0A register, so it won't compile.

Since Arduino IDE 1.6, Serial.flush() waits data on buffer to be transfered, and that is exactly what is being replaced.

these modifications are not tested yet

Author: gabrielsan

ModbusRtu.h

@@ -5,6 +5,7 @@
  * @author 	Samuel Marco i Armengol
  * @contact     [email protected]
  * @contribution Helium6072
+ * @contribution gabrielsan
  *
  * @description
  *  Arduino library for communicating with Modbus devices
@@ -896,73 +897,25 @@ void Modbus::sendTxBuffer()
     u8BufferSize++;
     au8Buffer[ u8BufferSize ] = u16crc & 0x00ff;
     u8BufferSize++;
-
-    // set RS485 transceiver to transmit mode
+	
     if (u8txenpin > 1)
     {
-        switch( u8serno )
-        {
-#if defined(UBRR1H)
-        case 1:
-            UCSR1A=UCSR1A |(1 << TXC1);
-            break;
-#endif
-
-#if defined(UBRR2H)
-        case 2:
-            UCSR2A=UCSR2A |(1 << TXC2);
-            break;
-#endif
-
-#if defined(UBRR3H)
-        case 3:
-            UCSR3A=UCSR3A |(1 << TXC3);
-            break;
-#endif
-        case 0:
-            UCSR0A=UCSR0A |(1 << TXC0);
-            break;
-        default:
-            break;
-        }
+        // set RS485 transceiver to transmit mode
         digitalWrite( u8txenpin, HIGH );
     }
 
     // transfer buffer to serial line
-    if(u8serno<4)
+    if (u8serno < 4)
         port->write( au8Buffer, u8BufferSize );
     else
         softPort->write( au8Buffer, u8BufferSize );
 
-    // keep RS485 transceiver in transmit mode as long as sending
     if (u8txenpin > 1)
     {
-        switch( u8serno )
-        {
-#if defined(UBRR1H)
-        case 1:
-            while (!(UCSR1A & (1 << TXC1)));
-            break;
-#endif
-
-#if defined(UBRR2H)
-        case 2:
-            while (!(UCSR2A & (1 << TXC2)));
-            break;
-#endif
-
-#if defined(UBRR3H)
-        case 3:
-            while (!(UCSR3A & (1 << TXC3)));
-            break;
-#endif
-        case 0:
-            while (!(UCSR0A & (1 << TXC0)));
-            break;
-        default:
-			break;
-        }
-
+        // must wait transmission end before changing pin state
+        // soft serial does not need it since it is blocking
+        if (u8serno < 4)
+            port->flush();
         // return RS485 transceiver to receive mode
         digitalWrite( u8txenpin, LOW );
     }