media: v4l2-dev: Make open and release file operations mandatory

media: v4l2-dev: Make open and release file operations mandatory

All V4L2 drivers implement the open and release file operations. As all
new drivers will need to use v4l2_fh, this situation won't change. Make
those two file operation mandatory at registration time. This allows
simplifying v4l2_open() and v4l2_release().

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>