usb: gadget: uvc: Factor out video USB request queueing

USB requests for video data are queued from two different locations in
the driver, with the same code block occurring twice. Factor it out to a
function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Tested-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>