sound: Address some XXX comments regarding AC'97 IDs

These seem harmless to address. Not sure why the original author did not
just assign the appropriate names if he knew they were wrong.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D56044

(cherry picked from commit edad981e1e8673b6dd14656fe75b9f239fb777ce)