USB: iowarrior: fix NULL-deref in write

Make sure to verify that we have the required interrupt-out endpoint for
IOWarrior56 devices to avoid dereferencing a NULL-pointer in write
should a malicious device lack such an endpoint.

Fixes: 946b960d ("USB: add driver for iowarrior devices.")
Cc: stable <stable@vger.kernel.org>     # 2.6.21
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>