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: de67f28abe58 ("iio: adc: ad4030: check scan_type for error")
Fixes: 949abd1ca5a4 ("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>