gpiolib: Assign fwnode to parent's if no primary one provided

In case when the properties are supplied in the secondary fwnode
(for example, built-in device properties) the fwnode pointer left
unassigned. This makes unable to retrieve them.

Assign fwnode to parent's if no primary one provided.

Fixes: 7cba1a4d ("gpiolib: generalize devprop_gpiochip_set_names() for device properties")
Fixes: 2afa97e9868f ("gpiolib: Read "gpio-line-names" from a firmware node")
Reported-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Tested-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>