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

commit 1bd303dc04c3f744474e77c153575087b657f7e1 upstream.

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: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>