GTP: refactor the echo timer

Move the check of the echo timer into an own function.
The gtp echo timer must be re-check everytime the
echo-timer has been modified or deactivated via vty.

Fixes the TTCN3 SGSN_Tests.TC_attach_restart_ctr_echo

Change-Id: Ia33471a9a9cfc3887facb665c82094b99932052a
diff --git a/src/gprs/sgsn_vty.c b/src/gprs/sgsn_vty.c
index de193f0..63985bc 100644
--- a/src/gprs/sgsn_vty.c
+++ b/src/gprs/sgsn_vty.c
@@ -383,6 +383,7 @@
 			     "not be lower than 60 seconds, use this value for " \
 			     "testing purposes only!%s", VTY_NEWLINE);
 
+	sgsn_ggsn_ctx_check_echo_timer(ggc);
 	return CMD_SUCCESS;
 }
 
@@ -395,6 +396,7 @@
 	struct sgsn_ggsn_ctx *ggc = sgsn_ggsn_ctx_find_alloc(id);
 
 	ggc->echo_interval = -1;
+	sgsn_ggsn_ctx_check_echo_timer(ggc);
 
 	return CMD_SUCCESS;
 }