GCS_MAVLink: report airspeed unhealthy when being rejected

this gives the pilot feedback on airspeed failing EKF innnovation
checks and switching to synthetic airspeed

Author: tridge

libraries/GCS_MAVLink/GCS.cpp

@@ -269,10 +269,12 @@ void GCS::update_sensor_status_flags()
     const AP_Airspeed *airspeed = AP_Airspeed::get_singleton();
     if (airspeed && airspeed->enabled()) {
         control_sensors_present |= MAV_SYS_STATUS_SENSOR_DIFFERENTIAL_PRESSURE;
-        if (airspeed->use()) {
+        const bool use = airspeed->use();
+        const bool enabled = AP::ahrs().airspeed_sensor_enabled();
+        if (use) {
             control_sensors_enabled |= MAV_SYS_STATUS_SENSOR_DIFFERENTIAL_PRESSURE;
         }
-        if (airspeed->all_healthy()) {
+        if (airspeed->all_healthy() && (!use || enabled)) {
             control_sensors_health |= MAV_SYS_STATUS_SENSOR_DIFFERENTIAL_PRESSURE;
         }
     }