scsi: fix warning

hw/scsi-bus.c:758: warning: ‘xfer’ may be used uninitialized in this
function

Isn't true, but older gcc versions (for example 4.1 as shipped in rhel5)
are not clever enougth to figure, so sprinkle in a default: line to make
them happy.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

hw/scsi-bus.c

@@ -761,6 +761,7 @@ static int ata_passthrough_12_xfer_size(SCSIDevice *dev, uint8_t *buf)
     switch (length) {
     case 0:
     case 3: /* USB-specific.  */
+    default:
         xfer = 0;
         break;
     case 1:
@@ -784,6 +785,7 @@ static int ata_passthrough_16_xfer_size(SCSIDevice *dev, uint8_t *buf)
     switch (length) {
     case 0:
     case 3: /* USB-specific.  */
+    default:
         xfer = 0;
         break;
     case 1: