[ALSA] AD1888 mixer controls for DC mode

This patch adds two mixer controls. The V_REFOUT enable is a
documented register that couples the microphone input lines
to the V_REFOUT DC source. The High Pass Filter enable in the
AC97_AD_TEST2 (0x5c) is an undocumented register provided by
Miller Puckette via Analog Devices that enables the AD codec
to apply a high pass filter to the input.
Signed-off-by: NJaya Kumar <jayakumar.alsa@gmail.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@suse.cz>

include/sound/ac97_codec.h

@@ -265,6 +265,7 @@
 
 /* specific - Analog Devices */
 #define AC97_AD_TEST		0x5a	/* test register */
+#define AC97_AD_TEST2		0x5c	/* undocumented test register 2 */
 #define AC97_AD_CODEC_CFG	0x70	/* codec configuration */
 #define AC97_AD_JACK_SPDIF	0x72	/* Jack Sense & S/PDIF */
 #define AC97_AD_SERIAL_CFG	0x74	/* Serial Configuration */

sound/pci/ac97/ac97_patch.c

@@ -1824,6 +1824,8 @@ static const struct snd_kcontrol_new snd_ac97_ad1888_controls[] = {
 		.get = snd_ac97_ad1888_lohpsel_get,
 		.put = snd_ac97_ad1888_lohpsel_put
 	},
+	AC97_SINGLE("V_REFOUT Enable", AC97_AD_MISC, 2, 1, 1),
+	AC97_SINGLE("High Pass Filter Enable", AC97_AD_TEST2, 12, 1, 1),
 	AC97_SINGLE("Spread Front to Surround and Center/LFE", AC97_AD_MISC, 7, 1, 0),
 	{
 		.iface = SNDRV_CTL_ELEM_IFACE_MIXER,