media: v4l2-async: Add notifier operation to destroy asd instances

Drivers typically extend the v4l2_async_subdev structure by embedding it
in a driver-specific structure, to store per-subdev custom data. The
v4l2_async_subdev instances are freed by the v4l2-async framework, which
makes this mechanism cumbersome to use safely when custom data needs
special treatment to be destroyed (such as freeing additional memory, or
releasing references to kernel objects).

To ease this, add a .destroy() operation to the
v4l2_async_notifier_operations structure. The operation is called right
before the v4l2_async_subdev is freed, giving drivers a chance to
destroy data if needed.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>