ASoC: rt5645: add DAC1 soft volume func control

This patch add an alsa control for DAC1 digital volume control function
selection. The options are:
0: Gain update immediately
1: Gain update when a zero crossing
2: Gain update when a zero crossing with a soft ramp

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/sound/soc/codecs/rt5645.h b/sound/soc/codecs/rt5645.h
index 205e071..cfc5f975 100644
--- a/sound/soc/codecs/rt5645.h
+++ b/sound/soc/codecs/rt5645.h
@@ -2018,6 +2018,9 @@
 
 
 /* Codec Private Register definition */
+/* DAC ADC Digital Volume (0x00) */
+#define RT5645_DA1_ZDET_SFT			6
+
 /* 3D Speaker Control (0x63) */
 #define RT5645_3D_SPK_MASK			(0x1 << 15)
 #define RT5645_3D_SPK_SFT			15