snd_dummy: Drain callout during detach

If we do not enter dummy_chan_trigger() before detaching, we'll get a
use-after-free since the callout(9) callback might be called after
having been detached.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D46715

(cherry picked from commit e42c8267821952407d4f4064026058aeaaa741ac)