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: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>