firmware: cs_dsp: Don't require client to provide a struct cs_dsp_client_ops

firmware: cs_dsp: Don't require client to provide a struct cs_dsp_client_ops

A client of cs_dsp does not necessarily need to implement any of the
optional callbacks in struct cs_dsp_client_ops, so allow the client_ops
pointer to be NULL.

This has been done by pointing client_ops at a default empty
cs_dsp_client_ops. It keeps the code cleaner by avoiding having to add
double nested NULL checks everywhere one of these callbacks is called.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20251128102132.1575177-2-rf@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>