usb: gadget: uvc: configfs: Use %u to print unsigned int values

The driver uses the %d format to print unsigned int values. The correct
format is %u. Fix it.

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>