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>