Add conditional channel count on Scarlett 18i20 version

alsa-project/alsa-ucm-conf pull request #660 was opened from dathegreat:

Resolves #559

Request URL : https://github.com/alsa-project/alsa-ucm-conf/pull/660 Patch URL : https://github.com/alsa-project/alsa-ucm-conf/pull/660.patch Repository URL: https://github.com/alsa-project/alsa-ucm-conf