[media] ad5820: unregister async sub-device

The async sub-device was not unregistered in ad5820_remove() as it should
have been; do it now. Also remove the now-redundant
v4l2_device_unregister_subdev().

Signed-off-by: Sakari Ailus <[e-mail]>
Acked-by: Pavel Machek <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/i2c/ad5820.c

···
 	struct v4l2_subdev *subdev = i2c_get_clientdata(client);
 	struct ad5820_device *coil = to_ad5820_device(subdev);
 
-	v4l2_device_unregister_subdev(&coil->subdev);
+	v4l2_async_unregister_subdev(&coil->subdev);
 	v4l2_ctrl_handler_free(&coil->ctrls);
 	media_entity_cleanup(&coil->subdev.entity);
 	mutex_destroy(&coil->power_lock);