Since commit 
1ce7b9349fad ("USB: serial: reuse generic write urb and
bulk-out buffer") the port write_urb is simply a pointer to the first
member of write_urbs so there's no need to kill it twice.
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
 
        if (serial->dev) {
                /* shutdown any bulk transfers that might be going on */
                if (port->bulk_out_size) {
-                       usb_kill_urb(port->write_urb);
                        for (i = 0; i < ARRAY_SIZE(port->write_urbs); ++i)
                                usb_kill_urb(port->write_urbs[i]);