MFC r359355:
Improve USB audio mixer support for USB audio class 1 and 2.

• make sure volume controls are correctly mapped to "pcm" and "rec" depending on how they deliver audio to the USB host.
• make sure there are no duplicate record selections.
• remove internal only mixer class type.
• don't add software volume controls for recording only.
• some minor mixer code cleanup.

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