gpio: cdev: open-code to_gpio_chardev_data()

This function is a wrapper around container_of(). It's used only once and
we will have a second notifier soon, so instead of having two flavors of
this helper, let's just open-code where needed.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Kent Gibson <warthog618@gmail.com>