media: usb: make video_device const

Make these const as they are only used during a copy operation.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>