V4L/DVB (8428): videodev: rename 'dev' to 'parent'

The field 'dev' is not the video device, but the parent of the video device.
Rename accordingly.

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