pipewire-pulse: improve channels and channel_map parsing

If we have channels and channelmap, make sure they match.
If we have only channel_map, use this to derive channels instead of
taking the default number of channels and then ending up with a
mismatch.
If we have only channels, use this to generate a channelmap from
predefined maps or just unknown channels instead of taking the default
channel map.
If neither channels or channelmap is defined, use the defaults.