gpio: f7188x: use gpiochip_get_data instead of container_of

gpiochip_add_data is already used to add data pointer and chip.
Lets rely on gpiochip_get_data which is getting used in other
gpio_chip functions.

Signed-off-by: Amitesh Singh <singh.amitesh@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>