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

Change-Id: I0ac198a49ba70ea40fea18464325f1925797a6e8
diff --git a/src/logging.c b/src/logging.c
index f81781d..3fa2a69 100644
--- a/src/logging.c
+++ b/src/logging.c
@@ -19,6 +19,13 @@
 		.color = "\033[1;33m",
 		.enabled = 1, .loglevel = LOGL_NOTICE,
 	},
+	[DSS] = {
+		.name = "DSS",
+		.description = "Supplementary Services",
+		.color = "\033[1;34m",
+		.enabled = 1, .loglevel = LOGL_NOTICE,
+	},
+
 };
 
 const struct log_info hlr_log_info = {