V4L/DVB (8482): videodev: move all ioctl callbacks to a new v4l2_ioctl_ops struct

All ioctl callbacks are now stored in a new v4l2_ioctl_ops struct. Drivers fill in
a const struct v4l2_ioctl_ops and video_device just contains a const pointer to it.

This ensures a clean separation between the const ops struct and the non-const
video_device struct.

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