net: lan78xx: advertise tx software timestamping support

lan78xx already calls skb_tx_timestamp() in its lan78xx_start_xmit().
Override .get_ts_info to also advertise this capability
(SOF_TIMESTAMPING_TX_SOFTWARE) via ethtool.

Signed-off-by: Markus Blöchl <markus.bloechl@ipetronik.com>
Signed-off-by: David S. Miller <davem@davemloft.net>