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/typhoon.c b/drivers/net/typhoon.c
index 3f4681f..5dfb395 100644
--- a/drivers/net/typhoon.c
+++ b/drivers/net/typhoon.c
@@ -1760,7 +1760,7 @@
 		   (TYPHOON_RX_IP_CHK_GOOD | TYPHOON_RX_UDP_CHK_GOOD)) {
 			new_skb->ip_summed = CHECKSUM_UNNECESSARY;
 		} else
-			new_skb->ip_summed = CHECKSUM_NONE;
+			skb_checksum_none_assert(new_skb);
 
 		spin_lock(&tp->state_lock);
 		if(tp->vlgrp != NULL && rx->rxStatus & TYPHOON_RX_VLAN)