pinctrl: single: Fix missing unlock on error path

Add the missing unlock before return from function
in the error handling case.

Fixes: 0f5972033509 ("pinctrl: single: Fix group and function selector use")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>