Plane: check both ground and airspeed in autocal test

when the airspeed ratio is far too low we were not raising it as the
airspeed was never getting above the minimum airspeed

Pair-Programmed-With: Jon Challinger

ArduPlane/ArduPlane.pde

@@ -1014,9 +1014,16 @@ static void airspeed_ratio_update(void)
 {
     if (!airspeed.enabled() ||
         g_gps->status() < GPS::GPS_OK_FIX_3D ||
-        g_gps->ground_speed_cm < 400 ||
-        airspeed.get_airspeed() < aparm.airspeed_min) {
+        g_gps->ground_speed_cm < 400) {
         // don't calibrate when not moving
+        return;        
+    }
+    if (airspeed.get_airspeed() < aparm.airspeed_min && 
+        g_gps->ground_speed_cm < (uint32_t)aparm.airspeed_min*100) {
+        // don't calibrate when flying below the minimum airspeed. We
+        // check both airspeed and ground speed to catch cases where
+        // the airspeed ratio is way too low, which could lead to it
+        // never coming up again
         return;
     }
     if (abs(ahrs.roll_sensor) > roll_limit_cd ||