[media] ivtv: replace ugly casts with a proper container_of

ivtv-ioctl cast the 'void *fh' directly to 'ivtv_open_id *'. This should be
done properly with a contained_of since the 'void *fh' is really a
'struct v4l2_fh *'.

It worked because the v4l2_fh field is also the first field in the ivtv_open_id
struct, but it is not clean code.

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