pinctrl: qcom: spmi-gpio: Correct power_source range check

Power source selection in DIG_VIN_CTL is indexed from 0, in the range
check it shouldn't be equal to the total number of power sources.

Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>