usb: core: urb: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1162594
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/urb.c

@@ -492,6 +492,7 @@ int usb_submit_urb(struct urb *urb, gfp_t mem_flags)
 			if ((urb->interval < 6)
 				&& (xfertype == USB_ENDPOINT_XFER_INT))
 				return -EINVAL;
+			/* fall through */
 		default:
 			if (urb->interval <= 0)
 				return -EINVAL;