gtp: make echo_interval unsigned

There's no real need to use -1 to indicate echo timer as disabled, since
0 can also be used (it doesn't make sense to have a timer timeout of 0).
This way code is simplified.

Change-Id: I689034887188a53590eddeffda781629694eb5ed
diff --git a/src/gprs/sgsn_vty.c b/src/gprs/sgsn_vty.c
index 68d3a77..42b5121 100644
--- a/src/gprs/sgsn_vty.c
+++ b/src/gprs/sgsn_vty.c
@@ -190,8 +190,8 @@
 			inet_ntoa(gctx->remote_addr), VTY_NEWLINE);
 		vty_out(vty, " ggsn %u gtp-version %u%s", gctx->id,
 			gctx->gtp_version, VTY_NEWLINE);
-		if (gctx->echo_interval != -1)
-			vty_out(vty, " ggsn %u echo-interval %"PRId32"%s",
+		if (gctx->echo_interval)
+			vty_out(vty, " ggsn %u echo-interval %u%s",
 				gctx->id, gctx->echo_interval, VTY_NEWLINE);
 		else
 			vty_out(vty, " ggsn %u no echo-interval%s",
@@ -395,7 +395,7 @@
 	uint32_t id = atoi(argv[0]);
 	struct sgsn_ggsn_ctx *ggc = sgsn_ggsn_ctx_find_alloc(id);
 
-	ggc->echo_interval = -1;
+	ggc->echo_interval = 0;
 	sgsn_ggsn_ctx_check_echo_timer(ggc);
 
 	return CMD_SUCCESS;