sound: Improve /dev/sndstat channel info readability

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, zlei
Differential Revision: https://reviews.freebsd.org/D46857

(cherry picked from commit 0ff0a8234f83e6b25e9a2f1b64623baaf723d6dc)