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

Change-Id: I0ac198a49ba70ea40fea18464325f1925797a6e8
diff --git a/src/logging.h b/src/logging.h
index fdaf5d1..ed24075 100644
--- a/src/logging.h
+++ b/src/logging.h
@@ -7,6 +7,7 @@
 	DDB,
 	DGSUP,
 	DAUC,
+	DSS,
 };
 
 extern const struct log_info hlr_log_info;