sound: Retire feeder_class->desc

The only field we care about is pcm_feederdesc->type, so keep that one
only and do not embed the whole pcm_feederdesc.

While here, make the feeder type enum into a named one and use it as
feeder_class->type's type.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53556