Improve USB audio mixer support for USB audio class 1 and 2.

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>
MFC after:	1 week
Sponsored by:	Mellanox Technologies