[nat] Fix VTY bug with access-lists...

vty->index does not hold a BSC Config at this point as we are
on the nat level... use the global _nat pointer for now...
diff --git a/openbsc/src/nat/bsc_nat_vty.c b/openbsc/src/nat/bsc_nat_vty.c
index e77a896..4b97ffc 100644
--- a/openbsc/src/nat/bsc_nat_vty.c
+++ b/openbsc/src/nat/bsc_nat_vty.c
@@ -387,9 +387,8 @@
       "The regexp of allowed IMSIs\n")
 {
 	struct bsc_nat_access_list *acc;
-	struct bsc_config *conf = vty->index;
 
-	acc = bsc_nat_accs_list_get(conf->nat, argv[0]);
+	acc = bsc_nat_accs_list_get(_nat, argv[0]);
 	if (!acc)
 		return CMD_WARNING;
 
@@ -405,9 +404,8 @@
       "The regexp of to be denied IMSIs\n")
 {
 	struct bsc_nat_access_list *acc;
-	struct bsc_config *conf = vty->index;
 
-	acc = bsc_nat_accs_list_get(conf->nat, argv[0]);
+	acc = bsc_nat_accs_list_get(_nat, argv[0]);
 	if (!acc)
 		return CMD_WARNING;