usb: gadget: uvc: constify vb2_ops structure

The vb2_ops structure can be const as it is only stored in the ops
field of a vb2_queue structure and this field is const.

Done with the help of Coccinelle.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>