enic: add sw timestamp support

Add ethtool ops to advertise sw timestamping.
Call skb_tx_timestamp() just before ringing the wq doorbell.

Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/cisco/enic/enic_main.c b/drivers/net/ethernet/cisco/enic/enic_main.c
index e130fb7..d98676e 100644
--- a/drivers/net/ethernet/cisco/enic/enic_main.c
+++ b/drivers/net/ethernet/cisco/enic/enic_main.c
@@ -856,6 +856,7 @@ static netdev_tx_t enic_hard_start_xmit(struct sk_buff *skb,
 
 	if (vnic_wq_desc_avail(wq) < MAX_SKB_FRAGS + ENIC_DESC_MAX_SPLITS)
 		netif_tx_stop_queue(txq);
+	skb_tx_timestamp(skb);
 	if (!skb->xmit_more || netif_xmit_stopped(txq))
 		vnic_wq_doorbell(wq);