pinctrl: qcom-spmi-mpp: Fix input value report

Fix interpretation of the pmic_mpp_read() return code,
negative value means an error.

Signed-off-by: Ivan T. Ivanov <ivan.ivanov@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>