gpio: Document proper return value for gpio drivers

The legacy defines GPIOF_DIR_XXX are only for consumers. Document the
proper ones.  Also: don't use "_XXX" since that's harder to find with
"git grep".  Just list both of the values.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200428172322.1.I396f351e364f3c09df7c7606e79abefb8682c092@changeid


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>