fix warning

common/usb_log.h

@@ -88,6 +88,8 @@ static inline void usb_hexdump(const void *ptr, uint32_t buflen)
     unsigned char *buf = (unsigned char *)ptr;
     uint32_t i, j;
 
+    (void)buf;
+
     for (i = 0; i < buflen; i += 16) {
         CONFIG_USB_PRINTF("%08X:", i);