Fix gateway RX indication

Added GW RX indication for MQTT and Serial gateway (PR #585).
Only send RX indication when valid message has been received.

core/MyGatewayTransportMQTTClient.cpp

@@ -131,6 +131,7 @@ void incomingMQTT(char* topic, byte* payload, unsigned int length) {
 				_MQTT_msg.set((const char*)payload);
 			}
 			_MQTT_available = true;
+            setIndication(INDICATION_GW_RX);
 		}
 		}
 		i++;