sound: Merge chn_intr() with chn_intr_locked()

There is no scenario where chn_intr() is called with the channel lock
already held.

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: kib, markj
Differential Revision: https://reviews.freebsd.org/D53854

(cherry picked from commit 0794365d5d3cc93940b873553382288a2a2c88c4)