sound: test PCM_REGISTERED before PCM_DETACHING

PCM_REGISTERED(d) tests that d is not NULL, so perform that check first
as we may have cases where devclass_get_softc has a null entry.

PR: 262671
Reviewed by: hselasky
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34648