gpio: ml-ioh: use gpiochip data pointer

This makes the driver use the data pointer added to the gpio_chip
to store a pointer to the state container instead of relying on
container_of().

Cc: Feng Tang <feng.tang@intel.com>
Cc: Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>