ASoC: wm_adsp: Support DSPs that don't require firmware download

When a DSP can self-boot from ROM it is not necessary to download
firmware - when the DSP has the wmfw_optional flag set not finding a
wmfw firmware file is a successful outcome and not an error condition.

Signed-off-by: Simon Trimmer <simont@opensource.cirrus.com>
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/167931140130.26.15590061696793062038@mailman-core.alsa-project.org
Signed-off-by: Mark Brown <broonie@kernel.org>