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>

sound/soc/fsl/fsl_ssi.c

@@ -157,6 +157,21 @@ static bool fsl_ssi_volatile_reg(struct device *dev, unsigned int reg)
 	}
 }
 
+static bool fsl_ssi_precious_reg(struct device *dev, unsigned int reg)
+{
+	switch (reg) {
+	case CCSR_SSI_SRX0:
+	case CCSR_SSI_SRX1:
+	case CCSR_SSI_SISR:
+	case CCSR_SSI_SACADD:
+	case CCSR_SSI_SACDAT:
+	case CCSR_SSI_SATAG:
+		return true;
+	default:
+		return false;
+	}
+}
+
 static bool fsl_ssi_writeable_reg(struct device *dev, unsigned int reg)
 {
 	switch (reg) {
@@ -179,6 +194,7 @@ static const struct regmap_config fsl_ssi_regconfig = {
 	.num_reg_defaults = ARRAY_SIZE(fsl_ssi_reg_defaults),
 	.readable_reg = fsl_ssi_readable_reg,
 	.volatile_reg = fsl_ssi_volatile_reg,
+	.precious_reg = fsl_ssi_precious_reg,
 	.writeable_reg = fsl_ssi_writeable_reg,
 	.cache_type = REGCACHE_RBTREE,
 };