pinctrl: qcom: spmi-mpp: Fix err handling of pmic_mpp_set_mux

'ret' should be returned while pmic_mpp_write_mode_ctl fails.

Fixes: 0e948042 ("pinctrl: qcom: spmi-mpp: Implement support for sink mode")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>