gsm_04_08.h: Clearly annotate timers that don't have a 3GPP Default value

Change-Id: I44fffaec1f7c0d819aa2ebc85e97f19581fc689c
diff --git a/include/osmocom/gsm/protocol/gsm_04_08.h b/include/osmocom/gsm/protocol/gsm_04_08.h
index 93b8c59..a7ab612 100644
--- a/include/osmocom/gsm/protocol/gsm_04_08.h
+++ b/include/osmocom/gsm/protocol/gsm_04_08.h
@@ -1495,14 +1495,14 @@
 #define GSM48_T303	30, 0
 #define GSM48_T305	30, 0
 #define GSM48_T306	30, 0
-#define GSM48_T308	10, 0
-#define GSM48_T310	180, 0
-#define GSM48_T313	30, 0
+#define GSM48_T308	10, 0		/* no spec default */
+#define GSM48_T310	180, 0		/* no spec default */
+#define GSM48_T313	30, 0		/* no spec default */
 #define GSM48_T323	30, 0
-#define GSM48_T331	30, 0
-#define GSM48_T333	30, 0
-#define GSM48_T334	25, 0 /* min 15 */
-#define GSM48_T338	30, 0
+#define GSM48_T331	30, 0		/* no spec default */
+#define GSM48_T333	30, 0		/* no spec default */
+#define GSM48_T334	25, 0 /* min 15s */
+#define GSM48_T338	30, 0		/* no spec default */
 #define GSM48_T303_MS	30, 0
 #define GSM48_T305_MS	30, 0
 #define GSM48_T308_MS	30, 0