media: v4l2-core: introduce a helper to unregister a i2c subdev

Introduce a new video4linux2 i2c helper, to unregister a subdev.
This allows to get rid of yet another ifdef.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: fix checkpatch warning]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>