staging:iio:dac:ad5791: Allow asymmetrical reference voltages

The ad5791 currently assumes that the negative and positive supply have the
same absolute value, which is not necessarily true. This patch introduces a
offset attribute which will contain the negative supply voltage scaled
according to the iio spec. The raw attribute now accepts values in the range
of 0 to max instead of -max/2 to max/2.

While we are at it also fix the vref span calculation. Since both positive and
negative reference voltages are specificed as absolute values we need to add
them and not subtract them to get the reference voltage span.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>