smsc95xx: use ethtool_op_get_ts_info()

This change enables the use of SW timestamping on Raspberry PI.

smsc95xx uses the usbnet transmit function usbnet_start_xmit(), which
implements software timestamping. However the SOF_TIMESTAMPING_TX_SOFTWARE
capability was missing and only SOF_TIMESTAMPING_RX_SOFTWARE was announced.
By using ethtool_op_get_ts_info() as get_ts_info() also the
SOF_TIMESTAMPING_TX_SOFTWARE is announced.

Signed-off-by: Petr Kulhavy <brain@jikos.cz>
Reviewed-by: Woojung Huh <Woojung.Huh@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>