hw/ac97: the volume mask is not only 0x1f

It's a case by case (see Table 66. AC ?97 Baseline Audio Register Map)
Signed-off-by: NMarc-Andr? Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Nmalc <av1474@comtv.ru>

hw/ac97.c

@@ -118,7 +118,6 @@ enum {
 #define EACS_VRA 1
 #define EACS_VRM 8
 
-#define VOL_MASK 0x1f
 #define MUTE_SHIFT 15
 
 #define REC_MASK 7