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

Read input buffer when input is enabled, not when it is
disabled. Also fix interpretation of the pmic_gpio_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>