ASoC: fsl_ssi: mark some registers precious

Mark some registers precious since their
reads have side effects (like clearing flags).

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>