staging: usbip: vhci: use urb->dev->portnum to find port

The access to pending_port was racy when two devices
were being attached at the same time.

Signed-off-by: Max Vozeler <max@vozeler.com>
Tested-by: Mark Wehby <MWehby@luxotticaRetail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>