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/dm9000.c b/drivers/net/dm9000.c
index 4fd6b2b..9f6aeef 100644
--- a/drivers/net/dm9000.c
+++ b/drivers/net/dm9000.c
@@ -1056,7 +1056,7 @@
 				if ((((rxbyte & 0x1c) << 3) & rxbyte) == 0)
 					skb->ip_summed = CHECKSUM_UNNECESSARY;
 				else
-					skb->ip_summed = CHECKSUM_NONE;
+					skb_checksum_none_assert(skb);
 			}
 			netif_rx(skb);
 			dev->stats.rx_packets++;