MAVLink: Use correct multi-instance syntax

src/modules/mavlink/mavlink_receiver.cpp

@@ -1178,7 +1178,8 @@ MavlinkReceiver::handle_message_heartbeat(mavlink_message_t *msg)
 			tstatus.heartbeat_time = tstatus.timestamp;
 
 			if (_telemetry_status_pub == nullptr) {
-				_telemetry_status_pub = orb_advertise_multi(ORB_ID(telemetry_status), &tstatus, NULL, ORB_PRIO_HIGH);
+				int multi_instance;
+				_telemetry_status_pub = orb_advertise_multi(ORB_ID(telemetry_status), &tstatus, multi_instance, ORB_PRIO_HIGH);
 
 			} else {
 				orb_publish(ORB_ID(telemetry_status), _telemetry_status_pub, &tstatus);