sound: Always use chn_getvolume_matrix()

There is no reason not to use it. We do it already with CHN_SETVOLUME().
chn_getvolume_matrix() is the same as the non-INVARIANTS
CHN_GETVOLUME(), just without the additional KASSERT
chn_getvolume_matrix() provides.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55990