[media] smiapp: Clean up media entity after unregistering subdev

media_entity_cleanup() frees the links array which will be accessed by
media_entity_remove_links() called by v4l2_device_unregister_subdev().

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>