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/b44.c b/drivers/net/b44.c
index 37617ab..7342308 100644
--- a/drivers/net/b44.c
+++ b/drivers/net/b44.c
@@ -818,7 +818,7 @@
 							 copy_skb->data, len);
 			skb = copy_skb;
 		}
-		skb->ip_summed = CHECKSUM_NONE;
+		skb_checksum_none_assert(skb);
 		skb->protocol = eth_type_trans(skb, bp->dev);
 		netif_receive_skb(skb);
 		received++;