staging:iio:ad7298: Fix temperature scale and offset

The temperature scale and offset depend on the reference voltage, the current
formula used in the driver assumes a 2.5V reference. This patch modifies the
code to report the unprocessed value for the temperature channel "raw" property
and to provide proper "scale" and "offset" properties which depend on the
selected reference voltage.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>