ir_nec: rephrase button lookup and addr/cmd validity checks

ir_nec: rephrase button lookup and addr/cmd validity checks

Eliminate redundancy in the check for database entries of button events.

Tighten the check for valid address and command bit patterns. The former
implementation did something unexpected, wanted the AND of the first and
second 8bit patterns to become zero. But the second 8bit item has not yet
become available when the test runs after the reception of the first 8bit
item. So the test happened to "pass" unexpectedly in intermediate steps,
and failed to catch invalid input data when all fields became available.

Unfortunately the check for valid data and the emission of annotations
was combined in the implementation. So it's essential to keep running
the "check" routine to get the annotations, and update internal ss/es
state. But only emit warnings when the check fails after all involved
data became available. Check for strict XOR as per the protocol spec.