Rover: use send_heartbeat() wrapper

APMrover2/GCS_Mavlink.cpp

@@ -66,10 +66,7 @@ void Rover::send_heartbeat(mavlink_channel_t chan)
     // indicate we have set a custom mode
     base_mode |= MAV_MODE_FLAG_CUSTOM_MODE_ENABLED;
 
-    mavlink_msg_heartbeat_send(
+    gcs[chan-MAVLINK_COMM_0].send_heartbeat(MAV_TYPE_GROUND_ROVER,
-        chan,
-        MAV_TYPE_GROUND_ROVER,
-        MAV_AUTOPILOT_ARDUPILOTMEGA,
                                             base_mode,
                                             custom_mode,
                                             system_status);