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/sb1250-mac.c b/drivers/net/sb1250-mac.c
index 8e6bd45..d8249d7 100644
--- a/drivers/net/sb1250-mac.c
+++ b/drivers/net/sb1250-mac.c
@@ -1170,7 +1170,7 @@
 						sb->ip_summed = CHECKSUM_UNNECESSARY;
 						/* don't need to set sb->csum */
 					} else {
-						sb->ip_summed = CHECKSUM_NONE;
+						skb_checksum_none_assert(sb);
 					}
 				}
 				prefetch(sb->data);