Fix to restore Gimbal control after Mission with ROI

ArduCopter/flight_mode.pde

@@ -213,6 +213,9 @@ static void exit_mode(uint8_t old_control_mode, uint8_t new_control_mode)
         if (mission.state() == AP_Mission::MISSION_RUNNING) {
             mission.stop();
         }
+#if MOUNT == ENABLED
+        camera_mount.set_mode_to_default();
+#endif  // MOUNT == ENABLED
     }
 
     // smooth throttle transition when switching from manual to automatic flight modes