ASoC: Automatically manage WM8900 sloping stopband filter

For best performance the DAC sloping stopband filter should be
enabled below 24kHz and not enabled above that so remove the
user visible control for this and do it autonomously in the
driver.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

sound/soc/codecs/wm8900.c

@@ -116,6 +116,7 @@
 #define WM8900_REG_CLOCKING2_DAC_CLKDIV 0x1c
 
 #define WM8900_REG_DACCTRL_MUTE          0x004
+#define WM8900_REG_DACCTRL_DAC_SB_FILT   0x100
 #define WM8900_REG_DACCTRL_AIF_LRCLKRATE 0x400
 
 #define WM8900_REG_AUDIO3_ADCLRC_DIR    0x0800
@@ -439,7 +440,6 @@ SOC_SINGLE("DAC Soft Mute Switch", WM8900_REG_DACCTRL, 6, 1, 1),
 SOC_ENUM("DAC Mute Rate", dac_mute_rate),
 SOC_SINGLE("DAC Mono Switch", WM8900_REG_DACCTRL, 9, 1, 0),
 SOC_ENUM("DAC Deemphasis", dac_deemphasis),
-SOC_SINGLE("DAC Sloping Stopband Filter Switch", WM8900_REG_DACCTRL, 8, 1, 0),
 SOC_SINGLE("DAC Sigma-Delta Modulator Clock Switch", WM8900_REG_DACCTRL,
 	   12, 1, 0),
 
@@ -743,6 +743,17 @@ static int wm8900_hw_params(struct snd_pcm_substream *substream,
 
 	wm8900_write(codec, WM8900_REG_AUDIO1, reg);
 
+	if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK) {
+		reg = wm8900_read(codec, WM8900_REG_DACCTRL);
+
+		if (params_rate(params) <= 24000)
+			reg |= WM8900_REG_DACCTRL_DAC_SB_FILT;
+		else
+			reg &= ~WM8900_REG_DACCTRL_DAC_SB_FILT;
+
+		wm8900_write(codec, WM8900_REG_DACCTRL, reg);
+	}
+
 	return 0;
 }