sound: Simplify pcm/feeder_mixer.c

• Get rid of macro magic.
• Make feed_mixer_info handling similar to most feeders.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48394

(cherry picked from commit 4021fa32d92d656d3d43186cc231695c7ad53d33)