usb: xhci: annotate switch/case fallthrough properly

The USB XHCI code uses an implicit switch/case fallthrough to share code
for handling full speed and low speed transfers.

Add our "fallthrough;" statement-like macro before the second label in
the XHCI code, to avoid a warning when GCC's -Wimplicit-fallthrough
warning option is enabled.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index d30725d3..3ee1f67 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -352,7 +352,7 @@
 		 * since it uses the same rules as low speed interrupt
 		 * endpoints.
 		 */
-
+		fallthrough;
 	case USB_SPEED_LOW:
 		if (usb_endpoint_xfer_int(endpt_desc) ||
 		    usb_endpoint_xfer_isoc(endpt_desc)) {