ASoC: wm_adsp: Use consistent error checks in wm_adsp_request_firmware_files()

ASoC: wm_adsp: Use consistent error checks in wm_adsp_request_firmware_files()

Use a consistent pattern of error checking in
wm_adsp_request_firmware_files().

- The integer return value of wm_adsp_request_firmware_file() reports
  unrecoverable errors, for example -ENOMEM.

- A NULL struct firmware pointer is a valid result. This not an error,
  not all DSPs require both files, some may not require any files.

Previously wm_adsp_request_firmware_files() was using a mix of checking
the return value and checking the struct firmware pointer to determine
whether a file was found. It wasn't checking for unrecoverable errors.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20260310141817.1871794-7-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>