Minor DT config fix, 7 is the common c_tag on the ONU/ONTs, not using it for the OLT s_tag

Change-Id: I2dac3d781548b96b742a34fb5804a409168004f2
diff --git a/configs/bbsim-sadis-dt.yaml b/configs/bbsim-sadis-dt.yaml
index d1133e3..8372b9c 100644
--- a/configs/bbsim-sadis-dt.yaml
+++ b/configs/bbsim-sadis-dt.yaml
@@ -6,6 +6,6 @@
       uni_tag_match: 4096
       c_tag: 4096
       c_tag_allocation: shared
-      s_tag: 7
+      s_tag: 900
       s_tag_allocation: unique
       tp_id: 64