ipg: use NULL, not zero, for pointers

Fixes a sparse warning in a code block that's hidden under JUMBO_FRAME #ifdef.
Tested-by: Andrew Savchenko <Bircoph@list.ru>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/net/ipg.c

@@ -1808,7 +1808,7 @@ static int ipg_nic_open(struct net_device *dev)
 	/* initialize JUMBO Frame control variable */
 	sp->jumbo.found_start = 0;
 	sp->jumbo.current_size = 0;
-	sp->jumbo.skb = 0;
+	sp->jumbo.skb = NULL;
 	dev->mtu = IPG_TXFRAG_SIZE;
 #endif