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/niu.c b/drivers/net/niu.c
index b4cc61f..1f89f47 100644
--- a/drivers/net/niu.c
+++ b/drivers/net/niu.c
@@ -3484,7 +3484,7 @@
 				     RCR_ENTRY_ERROR)))
 				skb->ip_summed = CHECKSUM_UNNECESSARY;
 			else
-				skb->ip_summed = CHECKSUM_NONE;
+				skb_checksum_none_assert(skb);
 		} else if (!(val & RCR_ENTRY_MULTI))
 			append_size = len - skb->len;