gpio: mockup: use value returning setters

gpio: mockup: use value returning setters

struct gpio_chip now has additional variants of the set(_multiple)
driver callbacks that return an integer to indicate success or failure.
Convert the driver to using them.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20250220-gpio-set-retval-v2-9-bc4cfd38dae3@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>