stmmac: enable transmit time stamping.

This patch enables software (and phy device) transmit time stamping
for the STMicroelectronics Ethernet driver. Compile tested only.

Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>

drivers/net/stmmac/stmmac_main.c

@@ -1081,6 +1081,8 @@ static netdev_tx_t stmmac_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	priv->hw->dma->enable_dma_transmission(priv->ioaddr);
 
+	skb_tx_timestamp(skb);
+
 	return NETDEV_TX_OK;
 }