[media] v4l: async: check for v4l2_dev in v4l2_async_notifier_register()

Add a check for v4l2_dev to v4l2_async_notifier_register() as to fail as
early as possible since this will fail later in v4l2_async_test_notify().

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