Plane: fixed a bug when switching to another mode from auto takeoff

when you exit auto and a takeoff is still in progress the flight stage
was not updated, which led to the throttle staying at maximum

Many thanks to Marco for finding this!

ArduPlane/ArduPlane.pde

@@ -1397,6 +1397,8 @@ static void update_alt()
             } else {
                 update_flight_stage(AP_SpdHgtControl::FLIGHT_NORMAL);
             }
+        } else {
+            update_flight_stage(AP_SpdHgtControl::FLIGHT_NORMAL);
         }
 
         SpdHgt_Controller->update_pitch_throttle(target_altitude_cm - home.alt + (int32_t(g.alt_offset)*100),