[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/eth_media.c b/net/tipc/eth_media.c
index 1f8d83b..7a25278 100644
--- a/net/tipc/eth_media.c
+++ b/net/tipc/eth_media.c
@@ -169,7 +169,7 @@
 
 static void disable_bearer(struct tipc_bearer *tb_ptr)
 {
-	((struct eth_bearer *)tb_ptr->usr_handle)->bearer = 0;
+	((struct eth_bearer *)tb_ptr->usr_handle)->bearer = NULL;
 }
 
 /**
@@ -285,7 +285,7 @@
 	for (i = 0; i < MAX_ETH_BEARERS ; i++) {
 		if (eth_bearers[i].bearer) {
 			eth_bearers[i].bearer->blocked = 1;
-			eth_bearers[i].bearer = 0;
+			eth_bearers[i].bearer = NULL;
 		}
 		if (eth_bearers[i].dev) {
 			dev_remove_pack(&eth_bearers[i].tipc_packet_type);