pinctrl: uniphier: fix WARN_ON() of pingroups dump on LD11

The pingroups dump of debugfs hits WARN_ON() in pinctrl_groups_show().
Filling non-existing ports with '-1' turned out a bad idea.

Fixes: 70f2f9c4 ("pinctrl: uniphier: add UniPhier PH1-LD11 pinctrl driver")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>