hfp: gateway_suspend_stream should trigger a state change

When gateway_suspend_stream is called it should change the
gateway state machine from PLAYING to CONNECTED.
diff --git a/audio/gateway.c b/audio/gateway.c
index 43a4b02..ec0ec5d 100644
--- a/audio/gateway.c
+++ b/audio/gateway.c
@@ -707,4 +707,5 @@
 	gw->sco = NULL;
 	gw->sco_start_cb = NULL;
 	gw->sco_start_cb_data = NULL;
+	change_state(dev, GATEWAY_STATE_CONNECTED);
 }