pinctrl: fix error path in pinconf_map_to_setting()

The code was using the union member
setting->data.configs.group_or_pin to store a potential
error code, but since that member is unsigned the
< 0 comparison was not true, letting errors pass through,
ending up as mapped to pin "-22". Fix this up and print
the error.

Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>