Have remaining drivers default to digits=2 for analog values.

The default so far was 0, which meant there would be no significant
digits at all, yielding results that looked strange/wrong to the user.

Long-term all remaining drivers should be fixed to use the actual,
correct digits and spec_digits values according to the device's
capabilities and/or datasheet/manual. Until that is done, a default
of digits=2 is used as a temporary workaround.

This fixes the remaining parts of bug #815.