[media] v4l: omap3isp: Fix async notifier registration order

The async notifier was registered before the v4l2_device was registered and
before the notifier callbacks were set. This could lead to missing the
bound() and complete() callbacks and to attempting to spin_lock() and
uninitialised spin lock.

Also fix unregistering the async notifier in the case of an error --- the
function may not fail anymore after the notifier is registered.

Fixes: da7f3843 ("[media] omap3isp: Add support for the Device Tree")

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>