[media] ad5820: unregister async sub-device

The async sub-device was not unregistered in ad5820_remove() as it should
have been; do it now. Also remove the now-redundant
v4l2_device_unregister_subdev().

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>