ASoC: cs-amp-lib-test: Use faux bus instead of creating a dummy platform device

ASoC: cs-amp-lib-test: Use faux bus instead of creating a dummy platform device

Change the code to use faux bus for the dummy codec driver device instead
of creating a platform device. Also use KUnit automatic resource cleanup to
destroy the device instead of doing it "manually" in a test case exit()
function.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20250416121604.780220-1-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>