Current implementation of the driver calls skb_tx_timestamp()to add a
software tx timestamp to the skb, however the software-transmit capability
is not reported in ethtool -T.

This commit updates the ethtool structure to report the software-transmit
capability in ethtool -T using the standard ethtool_op_get_ts_info().
This function reports all software timestamping capabilities (tx and rx),
as well as setting phc_index = -1. phc_index is the index of the PTP
hardware clock device that will be used for hardware timestamps. Since we
don't have such a device in ENA, using the default -1 value is the correct
setting.

Fixes: 1738cd3e ("net: ena: Add a driver for Amazon Elastic Network Adapters (ENA)")
Signed-off-by: Ezequiel Lara Gomez <ezegomez@amazon.com>
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>