V4L/DVB (8112): videodev: improve extended control support in video_ioctl2()

- add sanity checks for the extended controls argument.
- if the driver only supports extended controls, then convert
  old-style controls to an extended control callback.

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