iio: adc: move vadc_map_pt from header to the source file

struct vadc_map_pt is not used outside of qcom-vadc-common.c, so move it
there from the global header file.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20201204025509.1075506-9-dmitry.baryshkov@linaro.org


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