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
git-svn-id: svn://svn.freebsd.org/base/stable/10@359881
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