usb: gadget: uvc: fix permissions of configfs attributes

76e0da34 "usb-gadget/uvc: use per-attribute show and store methods"
removed write permission for writeable attributes. Correct attribute
permissions.

Fixes: 76e0da34 "usb-gadget/uvc: use per-attribute show and store methods"
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>