ASoC: rk3308_codec: add a bit of delay during enable DAC

The suggestion is from verndor, we should add a bit of
delay in key steps until the reference voltages are
stable, otherwise, the output of voltage maybe too low.

@HPMIX -6dB + LINEOUT -6dB:
normal Vpp: ~450mV
abnormal Vpp: ~100mV

Change-Id: I463d308b809b2c5e74e3bd37573bcbbd624df72e
Signed-off-by: Xing Zheng <zhengxing@rock-chips.com>

Author: acgzx

sound/soc/codecs/rk3308_codec.c

@@ -761,6 +761,9 @@ static int rk3308_codec_dac_enable(struct rk3308_codec_priv *rk3308)
 			   RK3308_DAC_BUF_REF_L_EN |
 			   RK3308_DAC_BUF_REF_R_EN);
 
+	/* Waiting the stable reference voltage */
+	udelay(10);
+
 	if (rk3308->dac_output == DAC_HPOUT ||
 	    rk3308->dac_output == DAC_LINEOUT_HPOUT) {
 		/* Step 03 */
@@ -778,6 +781,9 @@ static int rk3308_codec_dac_enable(struct rk3308_codec_priv *rk3308)
 			   RK3308_DAC_L_HPMIX_EN |
 			   RK3308_DAC_R_HPMIX_EN);
 
+	/* Waiting the stable HPMIX */
+	udelay(10);
+
 	/* Step 05 */
 	regmap_update_bits(rk3308->regmap, RK3308_DAC_ANA_CON13,
 			   RK3308_DAC_L_HPMIX_WORK |