ASoC: wm_adsp_fw_find_test: Clear searched_fw_files in find-by-index test

ASoC: wm_adsp_fw_find_test: Clear searched_fw_files in find-by-index test

In wm_adsp_fw_find_test_find_firmware_byindex() the content of
priv->searched_fw_files must be cleared before starting the next iteration.

The files searched for are appended to priv->searched_fw_files, so if it is
not cleared on each iteration it will still contain the searches from the
previous iteration.

Fixes: bf2d44d07de7 ("ASoC: wm_adsp: Add kunit test for firmware file search")
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20260505105123.3539778-3-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>