Rather than hold the topology lock across the whole handler, which
blocks detach of every device in the system, busy the one being selected
and used, and drop the topology lock, as the other SNDCTL_*INFO handlers
do.

The mutex now covers only the mixer fields. mixer->cdev is set once in
mixer_init() and never cleared, so reading does not need the lock.

MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation