gpio: etraxfs: use container_of() to get state container

The state container of the etraxfs GPIO driver is extracted from
the gpio_chip exploiting the fact that offsetof() the
struct gpio_chip inside the struct bgpio_chip are both 0, so
the container_of() is in practice a noop. However if a member
is added to struct etraxfs_gpio_chip in front of
struct bgpio_chip, things will break. Using proper container_of()
avoids this problem.

Semantically this is a noop, the compiler will optimize it away,
but syntactically it makes me happier.

Acked-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>