media: v4l: async: Register sub-devices before calling bound callback

Register the sub-device before calling the notifier's bound callback.
Doing this the other way around is problematic as the struct v4l2_device
has not assigned for the sub-device yet and may be required by the bound
callback.

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