pinctrl: uniphier: fix .pin_dbg_show() callback

Without this, reading the "pins" in the debugfs causes kernel BUG.

Fixes: 6e908892 ("pinctrl: UniPhier: add UniPhier pinctrl core support")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>