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

As per the API contract, the get_multiple() callback is only allowed to
return 0 or a negative error number. Filter out anything else.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250210-gpio-sanitize-retvals-v1-5-12ea88506cb2@linaro.org


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