sound: Move pcm_chnref() and pcm_chnrelease() to pcm/channel.c

Improve code layering. These are channel functions, and so they do not
belong in pcm/sound.c.

While here, assert in chn_ref() that new refcount won't be negative.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44985