sof-soundwire: use the ${find-device} lookup for the dmic device

[GitHub pull_request - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-ucm-conf pull request #77 was opened from perexg:

The PCM device number for the internal digital microphone is not fixed.

Request URL   : https://github.com/alsa-project/alsa-ucm-conf/pull/77
Patch URL     : https://github.com/alsa-project/alsa-ucm-conf/pull/77.patch
Repository URL: https://github.com/alsa-project/alsa-ucm-conf