sound: Improve sndstat nvlist feederchain format

• Use snd_afmt2str() to display format conversions in feeder_format, instead of the plain hex value.
• Simplify feeder_rate contents.
• Print "ch" (e.g 2.1ch) after matrix values in feeder_matrix.
• Use snd_afmt2str() instead of a plain hex for the the rest of the feeder classes.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D46309