sound: Move sndstat_register() call to pcm_setstatus()

The d->status string is populated in pcm_setstatus() anyway, so call
sndstat_register() after we populate it, and are closer to finalizing
the device creation.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D47323

(cherry picked from commit 181a31d8349088cbcead8dcbff8d62ee8af6c913)