stats: Fix handling of the no mtu command

For the atoi we need to pass the val as a string. This means
we need to write "0" which then gets parsed to 0.

[src/vty/stats_vty.c:90]: (error) Possible null pointer dereference: val
diff --git a/src/vty/stats_vty.c b/src/vty/stats_vty.c
index 0b7ac7a..98253ff 100644
--- a/src/vty/stats_vty.c
+++ b/src/vty/stats_vty.c
@@ -149,7 +149,7 @@
 	NO_STR "Set the maximum packet size\n")
 {
 	return set_srep_parameter_int(vty, osmo_stats_reporter_set_mtu,
-		0, "mtu");
+		"0", "mtu");
 }
 
 DEFUN(cfg_stats_reporter_prefix, cfg_stats_reporter_prefix_cmd,