usb: gadget: uvc: fix some error codes

We're basically saying ERR_CAST(NULL) and PTR_ERR(NULL) here, which is
nonsensical.

Acked-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>