iio: chemical: scd4x: Add a scale for the co2 concentration reading

This patch adds a scale for the co2 concentration reading. The reading is
expressed in percent after applying the scale to the raw value.

Signed-off-by: Roan van Dijk <roan@protonic.nl>
Link: https://lore.kernel.org/r/20211021140018.3051213-1-roan@protonic.nl


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