usb: gadget: missing fallthrough in composite_setup()

Add a missing fallthrough macro. This fixes a -Wimplicit-fallthrough
warning.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/drivers/usb/gadget/composite.c b/drivers/usb/gadget/composite.c
index 2a309e6..04b8541 100644
--- a/drivers/usb/gadget/composite.c
+++ b/drivers/usb/gadget/composite.c
@@ -1068,7 +1068,7 @@
 			if (!gadget_is_dualspeed(gadget) ||
 			    gadget->speed >= USB_SPEED_SUPER)
 				break;
-
+			fallthrough;
 		case USB_DT_CONFIG:
 			value = config_desc(cdev, w_value);
 			if (value >= 0)