media: : usb: add const to v4l2_file_operations structures

Declare v4l2_file_operations structures as const as they are only stored
in the fops field of video_device structures. This field is of type
const, so declare v4l2_file_operations structures with similar properties
as const.

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