ASoC: wm_adsp: Make cirrus_dir const

The cirrus_dir pointer should be const data but was missing the second
const needed to achieve this.

I haven't marked this as a 'Fixes' because it isn't causing any bugs,
it's only a code improvement.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20250522103816.543919-1-rf@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>