Fixed some issues for datalink loss & regain (GCS HEARTBEAT).

Major changes:
- Prevent  "HIGH LATENCY DATA LINK LOST" message to appear if iridium telemetry is not used.
- Prevent "DATA LINK LOSS" mavlink messages flooding when QGC is open and then closed.
- Changed "DATA LINK REGAINED" condition (use _datalink_last_heartbeat_gcs insthead of _datalink_lost)



Signed-off-by: Claudio Micheli <claudio@auterion.com>