mlx4: use reset to set mac header

Since offset is zero, it's not necessary to use set function. Reset
function is straightforward, and will remove the unnecessary add
operation in set function.
Signed-off-by: NZhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx4/en_selftest.c

@@ -68,7 +68,7 @@ static int mlx4_en_test_loopback_xmit(struct mlx4_en_priv *priv)
 	memcpy(ethh->h_dest, priv->dev->dev_addr, ETH_ALEN);
 	eth_zero_addr(ethh->h_source);
 	ethh->h_proto = htons(ETH_P_ARP);
-	skb_set_mac_header(skb, 0);
+	skb_reset_mac_header(skb);
 	for (i = 0; i < packet_size; ++i)	/* fill our packet */
 		packet[i] = (unsigned char)(i & 0xff);