i2cfilter: rephrase decoder implementation for maintainability

The previous implementation suffered from a severe issue (it kept
referencing caller's data beyond its validity) and from style issues
(redundant details, conditions scattered across distant locations).

Rephrase (actually rework) the decoder implementation to improve
readability as well as maintainability. Extend the TODO list while
the existing logic better lends itself to future extensions. Reword
comments while we are here. Some earlier constraints no longer apply
or were unfortunately phrased.