soc: qcom: socinfo: fix printing of pmic_model

Print sensible string instead of just "(null)" for unknown PMIC models.
Also as we are at it, do not let debugfs handler access past pmic_models
array.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20200525164817.2938638-2-dmitry.baryshkov@linaro.org
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>