adf435x: factor out inspection of 32bit words

Move the inspection of a completed 32bit word into the .decode_word()
routine. This simplifies checks for fatal constraints, as well as
optional inspection of deeper detail levels when extra parsers are
available.

Rename variables in the code path which accumulate bits. Add comments
to helper routines and to essential steps in complex interpretation
code paths. Eliminate the unused return value of the .decode_field()
method. Results in a lean and readable .decode() body.