USB: usb_wwan: kill interrupt urb explicitly at suspend

As the port interrupt URB is submitted by the subdriver at open, we
should also kill it explicitly at suspend (even though this will be
taken care of by USB serial core otherwise).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>