sound: Simplify pcm_feederdesc initialization

Currently we initialize a pcm_feederdesc array for every feeder, which
is then used by FEEDER_DECLARE(). However, there is no reason for this
to be an array, as each feeder has only one description. Additionally,
since the only thing we define in that array is the feeder type, remove
the pcm_feederdesc definitions altogether, and instead pass their type
to FEEDER_DECLARE() directly, which will then initialize the
pcm_feederdesc.

This also simplifies feeder_register().

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D53551