hp-3478a: fix values for digits properties

hp-3478a: fix values for digits properties

Unify the hp-3478a driver with the other drivers regarding the use of the
digits properties in the analog payload. This commit adjusts the previously
unclear, but now clarified, use of these properties:

Number of significant digits after the decimal point, if positive. When
negative, exponent with reversed polarity that is necessary to express the
value with all digits without a decimal point.