usb: host: xhci-mem: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.
Signed-off-by: NGustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/xhci-mem.c

@@ -1311,6 +1311,7 @@ static unsigned int xhci_get_endpoint_interval(struct usb_device *udev,
 		 * since it uses the same rules as low speed interrupt
 		 * endpoints.
 		 */
+		/* fall through */
 
 	case USB_SPEED_LOW:
 		if (usb_endpoint_xfer_int(&ep->desc) ||