[TIPC]: Fix simple sparse warnings

Tried to run the new tipc stack through sparse.
Following patch fixes all cases where 0 was used
as replacement of NULL.
Use NULL to document this is a pointer and to silence sparse.

This brough sparse warning count down with 127 to 24 warnings.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Per Liden <per.liden@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/tipc/node_subscr.c b/net/tipc/node_subscr.c
index afeea12..cff4068 100644
--- a/net/tipc/node_subscr.c
+++ b/net/tipc/node_subscr.c
@@ -47,7 +47,7 @@
 void tipc_nodesub_subscribe(struct node_subscr *node_sub, u32 addr, 
 		       void *usr_handle, net_ev_handler handle_down)
 {
-	node_sub->node = 0;
+	node_sub->node = NULL;
 	if (addr == tipc_own_addr)
 		return;
 	if (!tipc_addr_node_valid(addr)) {