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/ns83820.c b/drivers/net/ns83820.c
index 447c2c4..4475ca9 100644
--- a/drivers/net/ns83820.c
+++ b/drivers/net/ns83820.c
@@ -923,7 +923,7 @@
 			if ((extsts & 0x002a0000) && !(extsts & 0x00540000)) {
 				skb->ip_summed = CHECKSUM_UNNECESSARY;
 			} else {
-				skb->ip_summed = CHECKSUM_NONE;
+				skb_checksum_none_assert(skb);
 			}
 			skb->protocol = eth_type_trans(skb, ndev);
 #ifdef NS83820_VLAN_ACCEL_SUPPORT