rtl8139: correctly check the opmode

According to the spec, only when opmode is "Config. Register Write
Enable" could driver write to CONFIG0,1,3,4 and bits 13,12,8 of BMCR.

Currently, we allow modifying to those registers also when 8139 is in
"Auto-load" mode and "93C46 (93C56) Programming" mode. This patch
fixes this.
Signed-off-by: NJason Wang <jasowang@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

hw/rtl8139.c

@@ -332,8 +332,10 @@ enum CSCRBits {
 };
 
 enum Cfg9346Bits {
-    Cfg9346_Lock = 0x00,
-    Cfg9346_Unlock = 0xC0,
+    Cfg9346_Normal = 0x00,
+    Cfg9346_Autoload = 0x40,
+    Cfg9346_Programming = 0x80,
+    Cfg9346_ConfigWrite = 0xC0,
 };
 
 typedef enum {
@@ -1451,7 +1453,7 @@ static uint32_t rtl8139_IntrMitigate_read(RTL8139State *s)
 
 static int rtl8139_config_writable(RTL8139State *s)
 {
-    if (s->Cfg9346 & Cfg9346_Unlock)
+    if ((s->Cfg9346 & Chip9346_op_mask) == Cfg9346_ConfigWrite)
     {
         return 1;
     }