MFC r359446:
Add support for multiple playback and recording devices per physical USB audio
device. This requires some structural refactoring inside the driver, mostly
about converting existing audio channel structures into arrays.

The main audio mixer is provided by the first PCM instance.
The non-first audio instances may only have a software mixer for PCM playback.

Tested by: Horse Ma <Shichun.Ma@dell.com>
Sponsored by: Mellanox Technologies