ASoC: wm_adsp: Add KUnit redirection stubs for firmware file search

ASoC: wm_adsp: Add KUnit redirection stubs for firmware file search

Make some of the firmware file search functions redirectable for KUnit
testing.

- The call to firmware_request_nowarn() is factored out into a wrapper
  function so that it can be redirected.

- wm_adsp_request_firmware_files() and wm_adsp_release_firmware_files()
  are made visible and exported if KUNIT is enabled.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20260310141817.1871794-3-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>