nat: Attempt to follow the MODULE_METHOD pattern for methods

Rename methods to be like bsc_ussd_ACTION.
diff --git a/openbsc/include/openbsc/bsc_nat.h b/openbsc/include/openbsc/bsc_nat.h
index 6ebe045..635b125 100644
--- a/openbsc/include/openbsc/bsc_nat.h
+++ b/openbsc/include/openbsc/bsc_nat.h
@@ -437,8 +437,8 @@
 
 /** USSD filtering */
 int bsc_ussd_init(struct bsc_nat *nat);
-int bsc_check_ussd(struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed, struct msgb *msg);
-int bsc_close_ussd_connections(struct bsc_nat *nat);
+int bsc_ussd_check(struct nat_sccp_connection *con, struct bsc_nat_parsed *parsed, struct msgb *msg);
+int bsc_ussd_close_connections(struct bsc_nat *nat);
 
 struct msgb *bsc_nat_rewrite_msg(struct bsc_nat *nat, struct msgb *msg, struct bsc_nat_parsed *, const char *imsi);