media: ov772x: Unregister async subdevice

As the media subdevice is registered with 'v4l2_async_register_subdev()'
unregister it at module removal time with
'v4l2_async_unregister_subdev()'

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>