gpiolib: of: make fwnode take precedence in struct gpio_chip

If the driver sets the fwnode in struct gpio_chip, let it take
precedence over the of_node. This only affects OF-based systems,
ACPI needs to be converted separately.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>