USSD: Add new "DSS" logging category and use it appropriately

Change-Id: I0ac198a49ba70ea40fea18464325f1925797a6e8
diff --git a/doc/examples/osmo-hlr.cfg b/doc/examples/osmo-hlr.cfg
index cda1712..ebc22ec 100644
--- a/doc/examples/osmo-hlr.cfg
+++ b/doc/examples/osmo-hlr.cfg
@@ -11,6 +11,7 @@
   logging level main notice
   logging level db notice
   logging level auc notice
+  logging level ss info
   logging level linp error
 !
 line vty