Update ZgatewaySERIAL.ino Fix tx issues

The serial gateway can only receive serial data and publish via mqtt, but can not convert mqtt into serial data. It is because the receiverReady is not handled correctly for the tx part. A quick work around is to set receiverReady to true. (it seems not used)

main/ZgatewaySERIAL.ino

@@ -53,7 +53,7 @@ const TickType_t semaphoreTimeout = pdMS_TO_TICKS(1000); // 1 second timeout
 Stream* SERIALStream = NULL;
 //unsigned long msgCount = 0;
 
-bool receiverReady = false;
+bool receiverReady = true;
 unsigned long lastHeartbeatReceived = 0;
 unsigned long lastHeartbeatAckReceived = 0;
 unsigned long lastHeartbeatSent = 0;
@@ -372,4 +372,4 @@ void handleHeartbeat() {
     sendHeartbeatAck();
   }
 }
-#endif
+#endif