wan: make LAPB callbacks const

This is compile tested only.
Suggested by dumpster diving in PAX.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/lapb.h b/include/net/lapb.h
index 96cb5dd..fd2bf57 100644
--- a/include/net/lapb.h
+++ b/include/net/lapb.h
@@ -95,7 +95,7 @@
 	struct sk_buff_head	write_queue;
 	struct sk_buff_head	ack_queue;
 	unsigned char		window;
-	struct lapb_register_struct callbacks;
+	const struct lapb_register_struct *callbacks;
 
 	/* FRMR control information */
 	struct lapb_frame	frmr_data;