gpio: max732x: 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: Nicholas Krause <xerofoify@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Semen Protsenko <semen.protsenko@globallogic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>