ASoC: wm_adsp: Some improvements to firmware file

ASoC: wm_adsp: Some improvements to firmware file

Merge series from Richard Fitzgerald <rf@opensource.cirrus.com>:

This series makes some improvements to the code that searches for firmware
files.

Patch 1 is a trivial patch to remove an unused function argument, before
   adding any new code that uses this API.

Patches 2..4 add KUnit testing to prove that the subsequent changes don't
   break anything.

The remaining patches remove duplicated code and clean up some of the
implementation.