media: tw9910: Unregister subdevice with v4l2-async

As the tw9910 subdevice is registered through the v4l2-async framework,
use the v4l2-async provided function to register it.

Fixes: 7b20f325 ("media: i2c: tw9910: Remove soc_camera dependencies")

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>