snd_uaudio: Do not use pcm_channel->lock to protect uaudio_chan

This is a layering violation, but it also makes the control flow
confusing; snd_uaudio(4) should have its own lock to protect its
structures.

Fixes: 5cc34a83e1 ("Revert "sound: Merge chn_intr() with chn_intr_locked()"")
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54102

(cherry picked from commit c234740c9d34c2b953c6010f6a01b0aca9648c08)