sound: Call DSP_REGISTERED before PCM_DETACHING

DSP_REGISTERED first checks if the softc is not null, through
PCM_REGISTERED, which in turn calls PCM_ALIVE, whereas PCM_DETACHING
accesses the softc flags directly.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D47195