ASoC: rt5645: set RT5645_PRIV_INDEX as volatile

RT5645_PRIV_INDEX(0x6a) indicate the address of PR- registers. So,
it should be volatile.
Signed-off-by: NBard Liao <bardliao@realtek.com>
Signed-off-by: NMark Brown <broonie@kernel.org>

sound/soc/codecs/rt5645.c

@@ -440,6 +440,7 @@ static bool rt5645_volatile_register(struct device *dev, unsigned int reg)
 
 	switch (reg) {
 	case RT5645_RESET:
+	case RT5645_PRIV_INDEX:
 	case RT5645_PRIV_DATA:
 	case RT5645_IN1_CTRL1:
 	case RT5645_IN1_CTRL2: