gpio: sx150x: use container_of() to get state container

The state container of the sx150x GPIO driver is sometimes
extracted from the gpio_chip exploiting the fact that offsetof()
the struct gpio_chip inside the struct sx150x_chip is 0, so
the container_of() is in practice a noop. However if a member
is added to struct sx150_chip in front of struct gpio_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.

Cc: Wei Chen <Wei.Chen@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>