usb: gadget: f_uvc: make uvc_v4l2_fops const

Make this const as it is only stored in the const field of a structure
video_device in the file referencing it. Make the declaration const too.

Done using Coccinelle.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>