Improve message handling in mavlink IRIDIUM mode

- Do not send any mission updates as they are handled within HIGH_LATENCY2
- Always send a command acknowledge even if transmitting is not enabled

src/modules/mavlink/mavlink_mission.cpp

@@ -479,6 +479,11 @@ MavlinkMissionManager::send_mission_item_reached(uint16_t seq)
 void
 MavlinkMissionManager::send(const hrt_abstime now)
 {
+	// do not send anything over high latency communication
+	if (_mavlink->get_mode() == Mavlink::MAVLINK_MODE_IRIDIUM) {
+		return;
+	}
+
 	bool updated = false;
 	orb_check(_mission_result_sub, &updated);