smsc9420: enable transmit time stamping.

This patch enables software (and phy device) transmit time stamping
for the smsc9420. Compile tested only.

Cc: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: David S. Miller <davem@conan.davemloft.net>

drivers/net/smsc9420.c

@@ -1035,6 +1035,8 @@ static netdev_tx_t smsc9420_hard_start_xmit(struct sk_buff *skb,
 	smsc9420_reg_write(pd, TX_POLL_DEMAND, 1);
 	smsc9420_pci_flush_write(pd);
 
+	skb_tx_timestamp(skb);
+
 	return NETDEV_TX_OK;
 }