sound: Remove KASSERT from vchan_setnew()

This check is not really useful, and can in fact break things, if
sysctl_dev_pcm_vchans() calls vchan_setnew() with a value that will not
satisfy the KASSERT condition.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch, emaste
Differential Revision: https://reviews.freebsd.org/D46545

(cherry picked from commit 998de46c51452b046ecd9ba8ed592dd7c4f79f51)