media: v4l2-async: Document asd allocation requirements

media: v4l2-async: Document asd allocation requirements

The v4l2_async_notifier_add_subdev() function requires the asd pointer
it receives to be allocated dynamically, but doesn't explicitly say so.
Only one driver out of 13 get its right (atmel-sama5d2-isc.c, but with
memory leaks in the error paths), clearly showing we have an issue.

Update the v4l2_async_notifier_add_subdev() documentation to clearly
state the allocation requirement. Whether this will be enough to avoid
new offending code isn't certain, but it's a good first step
nonetheless.

Fixes: 9ca465312132 ("media: v4l: fwnode: Support generic parsing of graph endpoints in a device")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>