usb: usbip: vudc: Fix WARN_ON() usage pattern

Fix WARN_ON() macro usage as suggested by Felipe.
Instead of using:
if (cond) {
   WARN_ON(1);
   do_stuff();
}

Use a better pattern with WARN_ON() placed in if condition:

if (WARN_ON(cond))
   do_stuff();
Signed-off-by: NKrzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/usbip/vudc_dev.c

@@ -312,10 +312,9 @@ static void vep_free_request(struct usb_ep *_ep, struct usb_request *_req)
 {
 	struct vrequest *req;
 
-	if (!_ep || !_req) {
-		WARN_ON(1);
+	if (WARN_ON(!_ep || !_req))
 		return;
-	}
+
 	req = to_vrequest(_req);
 	kfree(req);
 }