iio: palmas: shut up warning about calibration mismatch (due to noise)

iio: palmas: shut up warning about calibration mismatch (due to noise)

Although technically checking for ADC values below 0 is correct,
because they are outside of the calibration values, there is usually
noise which spuriously fills the console log with error messages if
calculated input voltage gets close to 0V.

Ignore small negative calculated values, but clamp them to 0.

Signed-off-by: H. Nikolaus Schaller <hns@goldelico.com>
Link: https://lore.kernel.org/r/1cee45bfc3fa2ab59dcc17242fb52468035360a1.1646743982.git.hns@goldelico.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>