uart: Reduce redundancy in sample inspection (state machine)

uart: Reduce redundancy in sample inspection (state machine)

Factor out the logic which inspects samples that were provided by the
PD version 3 query API, and dispatches their processing depending on
the progress of UART frame inspection. "Unroll" a loop over the RX and
TX signals.

This commit replaces some complicated variable assignments by easier to
verify invocations.