ALSA: hda: Fix max PCM level to 0 dB for AD1981_HP

BugLink: https://bugs.launchpad.net/bugs/461062

The original reporter states that PCM maxes at +12 dB and results in
very bad distortion.  Cap PCM at 0 dB to resolve this symptom.
Signed-off-by: Daniel T Chen <crimsun@ubuntu.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_analog.c

@@ -1789,6 +1789,14 @@ static int patch_ad1981(struct hda_codec *codec)
 
 		codec->patch_ops.init = ad1981_hp_init;
 		codec->patch_ops.unsol_event = ad1981_hp_unsol_event;
+		/* set the upper-limit for mixer amp to 0dB for avoiding the
+		 * possible damage by overloading
+		 */
+		snd_hda_override_amp_caps(codec, 0x11, HDA_INPUT,
+					  (0x17 << AC_AMPCAP_OFFSET_SHIFT) |
+					  (0x17 << AC_AMPCAP_NUM_STEPS_SHIFT) |
+					  (0x05 << AC_AMPCAP_STEP_SIZE_SHIFT) |
+					  (1 << AC_AMPCAP_MUTE_SHIFT));
 		break;
 	case AD1981_THINKPAD:
 		spec->mixers[0] = ad1981_thinkpad_mixers;