ekf2-rng: do not continuously reset terrain in rng height ref

When the height reference datum is ground level (range finder height
ref), the terrain state is not updated through fusion and should stay at
a constant altitude.

src/modules/ekf2/EKF/terrain_control.cpp

@@ -63,13 +63,12 @@ void Ekf::controlTerrainFakeFusion()
 		initTerrain();
 	}
 
-	if (!_control_status.flags.in_air
-	    && !_control_status.flags.rng_terrain
-	    && !_control_status.flags.opt_flow_terrain) {
-
-		bool recent_terrain_aiding = isRecent(_time_last_terrain_fuse, (uint64_t)1e6);
+	if (!_control_status.flags.in_air) {
+		bool no_terrain_aiding = !_control_status.flags.rng_terrain
+					 && !_control_status.flags.opt_flow_terrain
+					 && isTimedOut(_time_last_terrain_fuse, (uint64_t)1e6);
 
-		if (_control_status.flags.vehicle_at_rest || !recent_terrain_aiding) {
+		if (no_terrain_aiding && (_height_sensor_ref != HeightSensor::RANGE)) {
 			initTerrain();
 		}
 	}