int usb_string_id(struct usb_composite_dev *cdev)
{
	if (cdev->next_string_id < 254) {
		/* string id 0 is reserved by USB spec for list of
		 * supported languages */
		/* 255 reserved as well? -- mina86 */
		cdev->next_string_id++;
		return cdev->next_string_id;
	}
	return -ENODEV;
}