sound: Remove macro magic from pcm/feeder_matrix.c

Turn the FEEDMATRIX_DECLARE macro into a single inline function
(feed_matrix_apply()). There is no reason to have this as a macro, it
only complicated the code. An advantage of this patch is that, because
we no longer call the functions created by the macro through function
pointers (apply field of feed_matrix_info), we can call
feed_matrix_apply() directly in feed_matrix_feed().

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48035