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

MFC r280322 and r280429:
The synchronisation value returned by the so-called feedback endpoint
appears to be too inaccurate that it can be used to synchronize the
playback data stream. If there is a recording endpoint associated with
the playback endpoint, use that instead. That means if the isochronous
OUT endpoint is asynchronus the USB audio driver will automatically
start recording, if possible, to get exact information about the
needed sample rate adjustments. In no recording endpoint is present,
no rate adaption will be done.

While at it fix an issue where the hardware buffer pointers don't get
reset at the first device PCM trigger.

Make some variables 32-bit to avoid problems with multithreading.

Use the feedback value from the synchronization endpoint as fallback
when there is no recording channel.

PR: 198444

MFC r280322 and r280429:
The synchronisation value returned by the so-called feedback endpoint
appears to be too inaccurate that it can be used to synchronize the
playback data stream. If there is a recording endpoint associated with
the playback endpoint, use that instead. That means if the isochronous
OUT endpoint is asynchronus the USB audio driver will automatically
start recording, if possible, to get exact information about the
needed sample rate adjustments. In no recording endpoint is present,
no rate adaption will be done.

While at it fix an issue where the hardware buffer pointers don't get
reset at the first device PCM trigger.

Make some variables 32-bit to avoid problems with multithreading.

Use the feedback value from the synchronization endpoint as fallback
when there is no recording channel.

PR: 198444