gpio: aggregator: restore the set_config operation

gpio: aggregator: restore the set_config operation

Restore the set_config operation, as it was lost during the refactoring of
the gpio-aggregator driver while creating the gpio forwarder library.

Fixes: b31c68fd851e7 ("gpio: aggregator: handle runtime registration of gpio_desc in gpiochip_fwd")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202509281206.a7334ae8-lkp@intel.com
Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20250929-gpio-aggregator-fix-set-config-callback-v1-1-39046e1da609@bootlin.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>