usb-storage: fix SYNCHRONIZE_CACHE

Commit 59310659 is incomplete,
we'll arrive in the scsi command complete callback in CSW state
and must handle that case correctly.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/dev-storage.c

@@ -247,6 +247,9 @@ static void usb_msd_command_complete(SCSIRequest *req, uint32_t status, size_t r
                the status read packet.  */
             usb_msd_send_status(s, p);
             s->mode = USB_MSDM_CBW;
+        } else if (s->mode == USB_MSDM_CSW) {
+            usb_msd_send_status(s, p);
+            s->mode = USB_MSDM_CBW;
         } else {
             if (s->data_len) {
                 int len = (p->iov.size - p->result);