gpio: mockup: Fix usage of new GPIO_LINE_DIRECTION

Restore the external behavior of gpio-mockup to what it was prior to the
change to using GPIO_LINE_DIRECTION.

Fixes: e42615ec

 ("gpio: Use new GPIO_LINE_DIRECTION")
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>