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>
diff --git a/drivers/net/ipg.c b/drivers/net/ipg.c
index dbb77e3..2c03f4e 100644
--- a/drivers/net/ipg.c
+++ b/drivers/net/ipg.c
@@ -1808,7 +1808,7 @@
 	/* 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