iio: adc: ad4030: Fix _scale value for common-mode channels

Previously, the driver always used the amount of precision bits of
differential input channels to provide the scale to mV. Though,
differential and common-mode voltage channels have different amount of
precision bits and the correct number of precision bits must be considered
to get to a proper mV scale factor for each one. Use channel specific
number of precision bits to provide the correct scale value for each
channel.

Fixes: de67f28 ("iio: adc: ad4030: check scan_type for error")
Fixes: 949abd1 ("iio: adc: ad4030: add averaging support")
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: machschmitt

drivers/iio/adc/ad4030.c

@@ -385,7 +385,7 @@ static int ad4030_get_chan_scale(struct iio_dev *indio_dev,
 	struct ad4030_state *st = iio_priv(indio_dev);
 	const struct iio_scan_type *scan_type;
 
-	scan_type = iio_get_current_scan_type(indio_dev, st->chip->channels);
+	scan_type = iio_get_current_scan_type(indio_dev, chan);
 	if (IS_ERR(scan_type))
 		return PTR_ERR(scan_type);