ucm: Don't log errors during normal operation

Merged Tanu Kaskinen requested to merge tanuk/pulseaudio:drop-log-level into master

It's completely normal to not have explicit channel configuration for stereo devices. In fact, the ALSA developers actively avoid configuring the channels for stereo devices.

I also dropped the word "duplex" from the messages, because "stereo duplex" implies bidirectionality, but most devices use one direction only.

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