gpio: mockup: fix indicated direction

Commit 3edfb7bd ("gpiolib: Show correct direction from the
beginning") fixed an existing issue but broke libgpiod tests by
changing the default direction of dummy lines to output.

We don't break user-space so make gpio-mockup behave as before.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>