pinctrl: single: correct argument for pinconf

pcs_pinconf_set() is always using "arg << shift" to configure two
parameters case. But pcs_add_conf2() didn't remove shift for config
argument. So correct it.

Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>