sound: Retire CHN_SETVOLUME() and use chn_setvolume_matrix()

CHN_SETVOLUME() is just a wrapped around chn_setvolume_matrix() anyway,
so use it directly.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55992