gsm_04_08.h: Reduce T310 default to 30s.

3GPP doesn't specify a network-side T310 default, but waiting for 180s
(3 minutes!) for the next message after CALL CONFIRMED is clearly way
too long and will just use radio resources for no good reason.

Change-Id: Ia52f9358bc86b23c72af9c80e2fff5cb0004b57a
Related: OS#2884
diff --git a/include/osmocom/gsm/protocol/gsm_04_08.h b/include/osmocom/gsm/protocol/gsm_04_08.h
index a7ab612..e54084f 100644
--- a/include/osmocom/gsm/protocol/gsm_04_08.h
+++ b/include/osmocom/gsm/protocol/gsm_04_08.h
@@ -1496,7 +1496,7 @@
 #define GSM48_T305	30, 0
 #define GSM48_T306	30, 0
 #define GSM48_T308	10, 0		/* no spec default */
-#define GSM48_T310	180, 0		/* no spec default */
+#define GSM48_T310	30, 0		/* no spec default */
 #define GSM48_T313	30, 0		/* no spec default */
 #define GSM48_T323	30, 0
 #define GSM48_T331	30, 0		/* no spec default */