sound: Add __assert_unreachable() to default cases in pcm_chn_add() and pcm_chn_remove()

We should normally never enter these cases.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: markj, emaste
Differential Revision: https://reviews.freebsd.org/D44994

(cherry picked from commit 76f95bae092b7353ff82b7a0056ca5801bb98f76)