[media] v4l: Replace enums with fixed-sized fields in public structure

The v4l2_mbus_framefmt structure will be part of the public userspace
API and used (albeit indirectly) as an ioctl argument. As such, its size
must be fixed across userspace ABIs.

Replace the v4l2_field and v4l2_colorspace enums by __u32 fields and add
padding for future enhancements.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>