ASoC: fsl_ssi: mark SACNT register volatile

SACNT register should be marked volatile since
its WR and RD bits are cleared by SSI after
completing the relevant operation.
This unbreaks AC'97 register access.

Fixes: 05cf2379 ("ASoC: fsl_ssi: Add driver suspend and resume to support MEGA Fast")

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>