gpio: samsung: 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: Paul Bolle <pebolle@tiscali.nl>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Acked-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>