The pinconf_generic_dump_one() function makes the assumption that
pin_config_group_get() should return -EINVAL and -ENOTSUPP just like
pin_config_get() does.  Document that so it's more obvious.
Signed-off-by: NDouglas Anderson <dianders@chromium.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>