sound: Wrap dsp_clone() and mixer_clone() with bus_topo_lock()

Make sure that the softc isn't freed in between the checks.

Sponsored by: The FreeBSD Foundation
MFC after; 1 day
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44991

(cherry picked from commit b18b990d8e7b15d25243d85ea22374dfdde8a18b)