gpio: npcm-sgpio: don't use legacy GPIO chip setters

gpio: npcm-sgpio: don't use legacy GPIO chip setters

We've converted this driver to using the new GPIO line value setters but
missed the instance where the legacy callback is accessed directly using
the function pointer. This will lead to a NULL-pointer dereference as
this pointer is no longer populated. Fix it.

Fixes: 0e1a8930c941 ("gpio: npcm-sgpio: use new GPIO line value setter callbacks")
Link: https://lore.kernel.org/r/20250618-gpio-mmio-fix-setter-v1-1-2578ffb77019@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>