pinctrl: uniphier: fix WARN_ON() of pingroups dump on LD20

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

Fixes: 336306ee ("pinctrl: uniphier: add UniPhier PH1-LD20 pinctrl driver")
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>