firmware: cs_dsp: Support DSPs that don't require firmware download

When a DSP can self-boot from ROM it is not necessary to download
firmware during the powering up sequence.

A DSP that required firmware download would fail in a previous
configuration step if firmware was not available.

Signed-off-by: Simon Trimmer <simont@opensource.cirrus.com>
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20230320112245.115720-4-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>