davinci_emac: enable transmit time stamping.

This patch enables software (and phy device) transmit time stamping
for the DaVinci EMAC driver. Tested together with the dp83640 PHY.

Cc: Anant Gole <anantgole@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>

drivers/net/davinci_emac.c

@@ -1090,6 +1090,7 @@ static int emac_dev_xmit(struct sk_buff *skb, struct net_device *ndev)
 			dev_err(emac_dev, "DaVinci EMAC: desc submit failed");
 		goto fail_tx;
 	}
+	skb_tx_timestamp(skb);
 
 	return NETDEV_TX_OK;