hw/ac97: Mask out the EAPD bit on Powerdown Ctrl/Stat writes

The Linux AC97 driver tests this bit to decide wether or not to show
an External amplifier toggle control.

This patch was also tested with a Windows XP guest without any issues.
Signed-off-by: NHans de Goede <hdegoede@redhat.com>
Signed-off-by: Nmalc <av1474@comtv.ru>

hw/ac97.c

@@ -603,7 +603,7 @@ static void nam_writew (void *opaque, uint32_t addr, uint32_t val)
         mixer_reset (s);
         break;
     case AC97_Powerdown_Ctrl_Stat:
-        val &= ~0xf;
+        val &= ~0x800f;
         val |= mixer_load (s, index) & 0xf;
         mixer_store (s, index, val);
         break;