gpio: mb86s7x: 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().

Acked-By: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>