snd_dummy: Fix callout(9) races

Use callout_init_mtx(9) to associate the callback with the driver's
lock. Also make sure the callout is stopped properly during detach.

While here, introduce a dummy_active() function to know when it's
appropriate to stop or not reschedule the callout.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47459

(cherry picked from commit 5bd08172b4150503c9cf60ffe3c97716c5bf6fa1)