only advertise range topic of px4flow board if it is the primary range device (#5390)

this allows selecting another range device such as a lidar by starting that app first

src/drivers/px4flow/px4flow.cpp

@@ -245,6 +245,7 @@ PX4FLOW::init()
 	/* get a publish handle on the range finder topic */
 	struct distance_sensor_s ds_report = {};
 
+	if (_class_instance == CLASS_DEVICE_PRIMARY) {
 		_distance_sensor_topic = orb_advertise_multi(ORB_ID(distance_sensor), &ds_report,
 					 &_orb_class_instance, ORB_PRIO_HIGH);
 
@@ -252,6 +253,10 @@ PX4FLOW::init()
 			DEVICE_LOG("failed to create distance_sensor object. Did you start uOrb?");
 		}
 
+	} else {
+		DEVICE_LOG("not primary range device, not advertising");
+	}
+
 	ret = OK;
 	/* sensor is ok, but we don't really know if it is within range */
 	_sensor_ok = true;