iio: adc: qcom-vadc-common: rewrite vadc7 die temp calculation

iio: adc: qcom-vadc-common: rewrite vadc7 die temp calculation

qcom_vadc7_scale_hw_calib_die_temp() uses a table format different from
the rest of volt/temp conversion functions in this file. Also the
conversion functions results in non-monothonic values conversion, which
seems wrong.

Rewrite qcom_vadc7_scale_hw_calib_die_temp() to use
qcom_vadc_map_voltage_temp() directly, like the rest of conversion
functions do.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201204025509.1075506-10-dmitry.baryshkov@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>