gpiolib: sanitize the return value of gpio_chip::set_config()

The return value of the set_config() callback may be propagated to
user-space. If a bad driver returns a positive number, it may confuse
user programs. Tighten the API contract and check for positive numbers
returned by GPIO controllers.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250210-gpio-sanitize-retvals-v1-3-12ea88506cb2@linaro.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>