Fix compile problems when enabling UDP

Using 
#define MY_USE_UDP 
Failied building on AVR (only worked on ESP)

Fixes mysensors/MySensors#369

Author: henrikekblad

libraries/MySensors/core/MyGatewayTransportEthernet.cpp

@@ -264,10 +264,16 @@ bool gatewayTransportAvailable()
 
 		if (packet_size) {
 			//debug(PSTR("UDP packet available. Size:%d\n"), packet_size);
-			_ethernetServer.read(inputString[0].string, MY_GATEWAY_MAX_RECEIVE_LENGTH);
-			_w5100_spi_en(false);
-			debug(PSTR("UDP packet received: %s\n"), inputString[0].string);
-			return protocolParse(_ethernetMsg, inputString[0].string);
+			#if defined(MY_GATEWAY_ESP8266)
+				_ethernetServer.read(inputString[0].string, MY_GATEWAY_MAX_RECEIVE_LENGTH);
+				debug(PSTR("UDP packet received: %s\n"), inputString[0].string);
+				return protocolParse(_ethernetMsg, inputString[0].string);
+			#else
+				_ethernetServer.read(inputString.string, MY_GATEWAY_MAX_RECEIVE_LENGTH);
+				_w5100_spi_en(false);
+				debug(PSTR("UDP packet received: %s\n"), inputString.string);
+				return protocolParse(_ethernetMsg, inputString.string);
+			#endif
 		}
 	#else
 		#if defined(MY_GATEWAY_ESP8266)

libraries/MySensors/examples/GatewayW5100/GatewayW5100.ino

@@ -69,7 +69,7 @@
 #define MY_RF24_CS_PIN 6
 
 // Enable to UDP          
-//#define MY_USE_UDP
+#define MY_USE_UDP
 
 #define MY_IP_ADDRESS 192,168,178,66   // If this is disabled, DHCP is used to retrieve address
 // Renewal period if using DHCP