sound: Retire CHN_GETMUTE() and use chn_getmute_matrix()
chn_getmute_matrix() does what CHN_GETMUTE() does, but with a few
additional checks.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55993
(cherry picked from commit d171d5b8b5acb145efb41a57a2a8f3cded59a484)