intel-hda: fix stream search

commit ba43d28916c4f51c19bd7366089155ce81bee058 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: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
diff --git a/hw/intel-hda.c b/hw/intel-hda.c
index 02def4c..10769e0 100644
--- a/hw/intel-hda.c
+++ b/hw/intel-hda.c
@@ -402,7 +402,7 @@
             break;
         }
     }
-    if (st == NULL) {
+    if (s == 4) {
         return false;
     }
     if (st->bpl == NULL) {