media: usbtv: constify static structs

Constify two static structs which are never modified to allow the
compiler to put them in read-only memory.

The only usage of norm_params is only read from it in
usbtv_configure_for_norm(). Making it const shrinks the resulting
ko-file with 300 bytes (tested with gcc 10).

The only usage of usbtv_ioctl_ops is to put its address to the ioctl_ops
field in the video_device struct. Making it const moves ~1kb to
read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>