ir_sirc: reduce smarts to improve maintenance (.wait() API change)

ir_sirc: reduce smarts to improve maintenance (.wait() API change)

Eliminate redundancy in time to snum conversions and vice versa. Don't
locally overload .wait() with unexpected semantics and neither change
its parameters nor return values. Give reviewers and maintainers a
chance to see what's happening when they inspect call sites. The
'signals' identifier is unusual for pin states, use the more common
'pins' instead for consistency with other decoders. Reflect when return
values are not used, so that readers need to juggle fewer details in
their head.