gpiolib: of: Don't use GPIO chip fwnode in of_gpiochip_*()

GPIO library should rely only on the GPIO device's fwnode.
Hence, replace GPIO chip fwnode usage by respective OF node
of the GPIO device.

JFYI, this is partial revert of 8afe8255 ("gpiolib: of:
Prepare of_gpiochip_add() / of_gpiochip_remove() for fwnode").

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Benjamin Tissoires <bentiss@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>