intel-hda: fix stream search

commit ba43d289 introduces a bug:
The stream-not-found case doesn't error out any more, instead the
code silently uses the first stream.  Fix it.
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/intel-hda.c

@@ -402,7 +402,7 @@ static bool intel_hda_xfer(HDACodecDevice *dev, uint32_t stnr, bool output,
             break;
         }
     }
-    if (st == NULL) {
+    if (s == 4) {
         return false;
     }
     if (st->bpl == NULL) {