sound: Get rid of pcm/sndstat.h and turn macros into regular code

There is no reason to have macros for this. Putting the code in
sndstat_prepare_pcm() directly makes it easier to work with it.

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44545

(cherry picked from commit 6d1cee1640e1c360917e483ece5ec1bde423b788)