libmsc/ussd.c: cosmetic: correct log level and message

Change-Id: If6c270fae54ce79c2708c99d7762ce6ff71f43d9
diff --git a/src/libmsc/ussd.c b/src/libmsc/ussd.c
index 748cfbb..ab59571 100644
--- a/src/libmsc/ussd.c
+++ b/src/libmsc/ussd.c
@@ -72,7 +72,8 @@
 	gh = msgb_l3(msg);
 	rc = gsm0480_decode_ss_request(gh, msgb_l3len(msg), &req);
 	if (!rc) {
-		DEBUGP(DMM, "Unhandled SS\n");
+		LOGP(DMM, LOGL_ERROR, "SS/USSD message parsing error, "
+			"rejecting request...\n");
 		rc = gsm0480_send_ussd_reject(conn, &req);
 		return rc;
 	}