media: v4l2: async: remove locking when initializing async notifier

There is no need to hold the list_lock when initializing the local
asd_list of a notifier. Remove the lock handling to simplify the code
and remove a potential LOCKDEP warning.

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