media: v4l: async: Properly re-initialise notifier entry in unregister

The notifier_entry of a notifier is not re-initialised after unregistering
the notifier. This leads to dangling pointers being left there so use
list_del_init() to return the notifier_entry an empty list.

Fixes: b8ec754a ("media: v4l: async: Set v4l2_device and subdev in async notifier init")
Cc: <stable@vger.kernel.org> # for 6.6 and later
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>