net: use CHECKSUM_NONE instead of magic number

Two places in the kernel were doing skb->ip_summed = 0.

Change both to skb->ip_summed = CHECKSUM_NONE, which is more readable.
Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ksz884x.c

@@ -4898,7 +4898,7 @@ static netdev_tx_t netdev_tx(struct sk_buff *skb, struct net_device *dev)
 				goto unlock;
 			}
 			skb_copy_and_csum_dev(org_skb, skb->data);
-			org_skb->ip_summed = 0;
+			org_skb->ip_summed = CHECKSUM_NONE;
 			skb->len = org_skb->len;
 			copy_old_skb(org_skb, skb);
 		}

net/ipv6/ip6mr.c

@@ -663,7 +663,7 @@ static int pim6_rcv(struct sk_buff *skb)
 	skb_pull(skb, (u8 *)encap - skb->data);
 	skb_reset_network_header(skb);
 	skb->protocol = htons(ETH_P_IPV6);
-	skb->ip_summed = 0;
+	skb->ip_summed = CHECKSUM_NONE;
 	skb->pkt_type = PACKET_HOST;
 
 	skb_tunnel_rx(skb, reg_dev);