media: staging/imx: Switch to v4l2_async_notifier_add_*_subdev

media: staging/imx: Switch to v4l2_async_notifier_add_*_subdev

Switch to v4l2_async_notifier_add_*_subdev() when adding async subdevs
to the imx-media root notifier. This removes the need to check for
an already added asd, since v4l2_async_notifier_add_*_subdev() does this
check. Also no need to allocate a subdevs array when registering the
root notifier, or keeping an internal master asd_list, since this is
moved to the notifier's asd_list.

Signed-off-by: Steve Longerbeam <[email protected]>
Signed-off-by: Sakari Ailus <[email protected]>
Signed-off-by: Mauro Carvalho Chehab <[email protected]>