drivers/net: avoid some skb->ip_summed initializations

fresh skbs have ip_summed set to CHECKSUM_NONE (0)

We can avoid setting again skb->ip_summed to CHECKSUM_NONE in drivers.

Introduce skb_checksum_none_assert() helper so that we keep this
assertion documented in driver sources.

Change most occurrences of :

skb->ip_summed = CHECKSUM_NONE;

by :

skb_checksum_none_assert(skb);

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/smsc911x.c b/drivers/net/smsc911x.c
index 0909ae9..13ddcd4 100644
--- a/drivers/net/smsc911x.c
+++ b/drivers/net/smsc911x.c
@@ -1048,7 +1048,7 @@
 		smsc911x_rx_readfifo(pdata, (unsigned int *)skb->head,
 				     pktwords);
 		skb->protocol = eth_type_trans(skb, dev);
-		skb->ip_summed = CHECKSUM_NONE;
+		skb_checksum_none_assert(skb);
 		netif_receive_skb(skb);
 
 		/* Update counters */