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

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

Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Ezra Savard <ezra.savard@xilinx.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Acked-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>