Cast some vendor-specific spell on VIA VT1708S codecs to:  - make analog input loopback work;  - get access to the mics boost controls.
Cast some vendor-specific spell on VIA VT1708S codecs to:
 - make analog input loopback work;
 - get access to the mics boost controls.

Sponsored by:	iXsystems, Inc.
MFC after:	1 month

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