sink: Reduce chat sink priority

Some gaming sound cards have custom profiles with analog-game and analog-chat mappings that exist simultaneously. The game sink should have higher priority than the chat sink, but currently there's no way to affect the sink priorities from the profile-set configuration (the mapping priority is not propagated to the sink priority).

I first thought about adding the mapping priority to the sink priority, but that could mess up the prioritization system in pa_device_init_priority(). Since we already modify priorities based on device names, I added a check for "-chat" suffix in the code. I also reduced the iec958 priority so that the chat and iec958 sinks don't end up with the same priority.

Fixes: #818 (closed)