usb: dwc3: debug: fix sparse warning

Fix the following sparse warning:

drivers/usb/dwc3/trace.c: note: in included file (through drivers/usb/dwc3/trace.h):
drivers/usb/dwc3/debug.h:374:39: warning: cast to non-scalar
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/dwc3/debug.h

@@ -371,7 +371,9 @@ static inline const char *dwc3_gadget_event_type_string(u8 event)
 static inline const char *dwc3_decode_event(char *str, size_t size, u32 event,
 		u32 ep0state)
 {
-	const union dwc3_event evt = (union dwc3_event) event;
+	union dwc3_event evt;
+
+	memcpy(&evt, &event, sizeof(event));
 
 	if (evt.type.is_devspec)
 		return dwc3_gadget_event_string(str, size, &evt.devt);