sound: Bail out early if a format is not supported

If a format is unsupported, the feeder_build_* functions will fail
anyway, so bail out early to avoid unnecessary computation.

This is also needed by a series of upcoming patches to the feeder
framework.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48031

(cherry picked from commit 18457e7e25c55b8834e886fd925ec3b11b79d1b6)