gpiolib: Do not alter GPIO chip fwnode member

Ideally we should not touch data in the given GPIO chip structure.
Let's become closer to it by avoiding altering fwnode member.

The GPIO library must use fwnode from GPIO device and the drivers
might use one from GPIO chip in case they initialized it.

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>