The state container of the vf610 GPIO driver is sometimes
extracted from the gpio_chip exploiting the fact that offsetof()
the struct gpio_chip inside the struct vf610_gpio_port is 0, so
the container_of() is in practice a noop. However if a member
is added to struct vf610_gpio_port 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.

Also replace some explicit container_of() calls with the helper
function.
Acked-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>