mixer(3): Do not skip devices with no volume control

Some devices might not have a software volume control, but still be
able to be set as recording sources.

PR: 279787
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D48730

(cherry picked from commit f121de8fbcbc8aa82f9fb59039c502406b7c88b6)