ALSA: hda - Fix missing Master volume for STAC9200/925x

With the commit [2faa3bf1: ALSA: hda - Rewrite the mute-LED hook with
vmaster hook in patch_sigmatel.c], the former Master volume control
was converted to PCM.  This was supposed to be covered by the vmaster
control.  But due to the lack of "PCM" slave definition, this didn't
happen properly.  The patch fixes the missing entry.
Reported-by: Andrew Shadura <bugzilla@tut.by>
Cc: <stable@vger.kernel.org> [v3.4+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/patch_sigmatel.c

@@ -1075,7 +1075,7 @@ static struct snd_kcontrol_new stac_smux_mixer = {
 
 static const char * const slave_pfxs[] = {
 	"Front", "Surround", "Center", "LFE", "Side",
-	"Headphone", "Speaker", "IEC958",
+	"Headphone", "Speaker", "IEC958", "PCM",
 	NULL
 };