V4L/DVB (10644): v4l2-subdev: rename dev field to v4l2_dev

Remain consistent in the naming: fields pointing to v4l2_device should
be called v4l2_dev. There are too many device-like entities without
adding to the confusion by mixing naming conventions.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>