pinctrl: single: fix error return code in pcs_parse_one_pinctrl_entry()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Introduced by commit 9dddb4df
(pinctrl: single: support generic pinconf)

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