LandDetector: call param_notify_changes() instead of param_save_default()

param_save_default() could take something like 0.5s, and because the
LandDetector is running on the HP work queue, this would block other
tasks, like RC handling or drivers.

src/modules/land_detector/LandDetector.cpp

@@ -152,7 +152,7 @@ void LandDetector::_cycle()
 			param_set_no_notification(_p_total_flight_time_high, &flight_time);
 			flight_time = _total_flight_time & 0xffffffff;
 			param_set_no_notification(_p_total_flight_time_low, &flight_time);
-			param_save_default();
+			param_notify_changes(); // this will notify the commander, who will save the params
 		}
 
 		_landDetected.timestamp = hrt_absolute_time();