sound: Retire CHN_GETVOLUME() and use chn_getvolume_matrix()

CHN_GETVOLUME() is just a wrapped around chn_getvolume_matrix() anyway,
so use it directly.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55991