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: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/sound/soc/codecs/rt5645.c b/sound/soc/codecs/rt5645.c
index 3c6594d..c0438e4 100644
--- a/sound/soc/codecs/rt5645.c
+++ b/sound/soc/codecs/rt5645.c
@@ -440,6 +440,7 @@
 
 	switch (reg) {
 	case RT5645_RESET:
+	case RT5645_PRIV_INDEX:
 	case RT5645_PRIV_DATA:
 	case RT5645_IN1_CTRL1:
 	case RT5645_IN1_CTRL2: