sound: Use bus_topo_lock() where appropriate

Lock around uses of devclass_*() and replace leftover
CTLFLAG_NEEDGIANTs with CTLFLAG_MPSAFE.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: imp, jhb
Differential Revision: https://reviews.freebsd.org/D46700

(cherry picked from commit 35400672df83e337f8792df1972a15003b603930)