sound: Simplify vchan_create() error paths

Instead of checking the value of "ret" multiple times, just set a goto
label and jump there immediately in case of an error.

While here, remove a redundant assignment to "d".

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, markj, emaste
Differential Revision: https://reviews.freebsd.org/D46833

(cherry picked from commit 3cab66d12d439357b948093756ca1af87967c8cc)