media: v4l2-async: Get fwnode reference when putting it to the notifier's list

The v4l2_async_notifier_add_fwnode_subdev() did not take a reference of
the added fwnode, relying on the caller to handle that instead, in essence
putting the fwnode to be added if there was an error.

As the reference is eventually released during the notifier cleanup, this
is not intuitive nor logical. Improve this by always getting a reference
when the function succeeds, and the caller releasing the reference when it
does not *itself* need it anymore.

Luckily, perhaps, there were just a handful of callers using the function.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Tested-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>