usb: wusbcore: rename urb to tr_urb in struct wa_seg

Rename urb to tr_urb in struct wa_seg to make it clear that the urb is
used for the transfer request.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>