sound: Detect unsupported formats

This way we can avoid edge-cases like
8af6aee96ed609456900c6dd92dafabac5e89c0a ("virtual_oss(8): Remove
floating point formats from preference list").

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D55403