sound: Merge PCM_ALIVE() with PCM_REGISTERED()

PCM_ALIVE() is used only in pcm_unregister(), but it does not hurt to
use PCM_REGISTERED(), which uses PCM_ALIVE() internally. In fact, it's
more robust this way.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week

(cherry picked from commit 3107b952f534813846e4f58afdb57624a81618d8)