ALSA: hda - Rename power_mgmt flag with power_save_node

David suggested that the name "power_mgmt" is too ambiguous.  Rename
the flag with a bit clearer one "power_save_node".

Also, add the corresponding description to HD-Audio.txt, too.

Reported-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/pci/hda/patch_via.c b/sound/pci/hda/patch_via.c
index d5d1dca..485663b 100644
--- a/sound/pci/hda/patch_via.c
+++ b/sound/pci/hda/patch_via.c
@@ -133,7 +133,7 @@
 	spec->gen.keep_eapd_on = 1;
 	spec->gen.pcm_playback_hook = via_playback_pcm_hook;
 	spec->gen.add_stereo_mix_input = HDA_HINT_STEREO_MIX_AUTO;
-	codec->power_mgmt = 1;
+	codec->power_save_node = 1;
 	spec->gen.power_down_unused = 1;
 	return spec;
 }
@@ -236,7 +236,7 @@
 				 struct snd_ctl_elem_value *ucontrol)
 {
 	struct hda_codec *codec = snd_kcontrol_chip(kcontrol);
-	ucontrol->value.enumerated.item[0] = codec->power_mgmt;
+	ucontrol->value.enumerated.item[0] = codec->power_save_node;
 	return 0;
 }
 
@@ -247,9 +247,9 @@
 	struct via_spec *spec = codec->spec;
 	bool val = !!ucontrol->value.enumerated.item[0];
 
-	if (val == codec->power_mgmt)
+	if (val == codec->power_save_node)
 		return 0;
-	codec->power_mgmt = val;
+	codec->power_save_node = val;
 	spec->gen.power_down_unused = val;
 	analog_low_current_mode(codec);
 	return 1;
@@ -295,7 +295,7 @@
 	bool enable;
 	unsigned int verb, parm;
 
-	if (!codec->power_mgmt)
+	if (!codec->power_save_node)
 		enable = false;
 	else
 		enable = is_aa_path_mute(codec) && !spec->gen.active_streams;
@@ -517,7 +517,7 @@
 		return err;
 
 	/* disable widget PM at start for compatibility */
-	codec->power_mgmt = 0;
+	codec->power_save_node = 0;
 	spec->gen.power_down_unused = 0;
 	return 0;
 }