gpiolib: Fix gpio_direction_* for single direction GPIOs

GPIOs with no programmable direction are not required to implement
direction_output nor direction_input.

If we try to set an output direction on an output-only GPIO or input
direction on an input-only GPIO simply return 0.

This allows this single direction GPIO to be used by libgpiod.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>