airspeed driver: Do not spam the console on error, report the system change only once

src/drivers/airspeed/airspeed.h

@@ -118,14 +118,21 @@ protected:
 	virtual int	measure() = 0;
 	virtual int	collect() = 0;
 
+	/**
+	 * Update the subsystem status
+	 */
+	void update_status();
+
 	work_s			_work;
 	float			_max_differential_pressure_pa;
 	bool			_sensor_ok;
+	bool			_last_published_sensor_ok;
 	int			_measure_ticks;
 	bool			_collect_phase;
 	float			_diff_pres_offset;
 
 	orb_advert_t		_airspeed_pub;
+	orb_advert_t		_subsys_pub;
 
 	int			_class_instance;