sound: Retire feedertab_entry

Have an SLIST of feeder_class directly. This way we simplify the code,
and also avoid the additional malloc()/free() for each entry.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53554

(cherry picked from commit c24ca02c4c85d0b691a6100fd3006b1f23061858)