gpio: gpiolib: Simplify gpiochip_add_data_with_key() fwnode

The code defaulting to the parents fwnode if no fwnode was assigned
is unnecessarily convoluted, probably due to refactoring. Simplify
it and make it more human-readable.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Anders Roxell <anders.roxell@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>