pinctrl: core: fix missing unlock on error in pinctrl_find_gpio_range_from_pin()

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

Introduced by commit 2ff3477efd7086544b9e298fc63afab0645921b4.
(pinctrl: add pin list based GPIO ranges)

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