gpio: mmio: don't use legacy GPIO chip setters

gpio: mmio: don't use legacy GPIO chip setters

We've converted this driver to using the new GPIO line value setters but
missed the instances 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. The issue needs fixing locally as
well as in the already converted previously users of gpio-mmio.

Fixes: b908d35d0003 ("gpio: mmio: use new GPIO line value setter callbacks")
Reported-by: Klara Modin <klarasmodin@gmail.com>
Closes: https://lore.kernel.org/all/2rw2sncevdiyirpdovotztlg77apcq2btzytuv5jnm55aqhlne@swtts3hl53tw/
Tested-by: Klara Modin <klarasmodin@gmail.com>
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Tested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250618-gpio-mmio-fix-setter-v1-2-2578ffb77019@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>