iio: adc: qcom-vadc-common: use fixp_linear_interpolate

Use new function fixp_linear_interpolate() instead of hand-coding the
linear interpolation.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201204025509.1075506-5-dmitry.baryshkov@linaro.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>