V4L/DVB (8429): videodev: renamed 'class_dev' to 'dev'

The class_dev field is a normal device, not a class device. This is very
confusing and now that the old 'dev' field has been renamed to 'parent'
we can rename 'class_dev' to just 'dev'.

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