media: ad5820: Define entity function

Without this patch, media_device_register_entity throws a warning:

dev_warn(mdev->dev,
	 "Entity type for entity %s was not initialized!\n",
	 entity->name);

Signed-off-by: Ricardo Ribalda Delgado <ribalda@kernel.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>