net:phy:dp83640: Declare that TX timestamping possible

Set the SKBTX_IN_PROGRESS bit in tx_flags dp83640_txtstamp when doing
tx timestamps as per Documentation/networking/timestamping.txt.
Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/dp83640.c

@@ -1281,6 +1281,7 @@ static void dp83640_txtstamp(struct phy_device *phydev,
 		}
 		/* fall through */
 	case HWTSTAMP_TX_ON:
+		skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
 		skb_queue_tail(&dp83640->tx_queue, skb);
 		schedule_work(&dp83640->ts_work);
 		break;