media: v4l: async: fix unbind error in v4l2_async_notifier_unregister()

The call to v4l2_async_cleanup() will set sd->asd to NULL so passing it to
notifier->unbind() has no effect and leaves the notifier confused. Call
the unbind() callback prior to cleaning up the subdevice to avoid this.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>