stm32/usbdev: Change static function variable to non-static.

It's written straight away in the function on every call so it doesn't need
to be static.

ports/stm32/usbdev/core/src/usbd_ctlreq.c

@@ -479,7 +479,7 @@ static void USBD_SetConfig(USBD_HandleTypeDef *pdev ,
                            USBD_SetupReqTypedef *req)
 {
   
-  static uint8_t  cfgidx;
+  uint8_t cfgidx;
   
   cfgidx = (uint8_t)(req->wValue);