pinctrl: fix missing unlock on error in pinctrl_groups_show()

Add the missing unlock on the error handle path in function
pinctrl_groups_show().

Cc: stable@kernel.org
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>