sound: Change PCMDIR_* numbering

Needed by a follow-up patch, so that channels can be sorted properly.
This change does not affect the behavior of any subsystem.

While here, change to an enum.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47198