sound: Fix OSS API requests for more than 8 channels

Audio devices with more than 8 channels need bitperfect mode to operate,
the vchan processing chain is limited to 8 channels. For these devices,
let applications properly select a certain number of channels supported
by the driver, instead of mapping the request to a vchan format.

Reviewed by: emaste
Pull Request: https://github.com/freebsd/freebsd-src/pull/914