pinctrl/abx500: replace incorrect return value

Currently in the empty abx500_pin_config_get() function, we're
returning -EINVAL, with a comment stating that the reason for the
failure is that the function isn't implemented yet. Well there's
a proper return code for that. If we use it, we can do away with
the comment too, as it would be implied.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>