This enables the use of SW timestamping.

ax88179_178a uses the usbnet transmit function usbnet_start_xmit() which
implements software timestamping. ax88179_178a overrides ethtool_ops but
missed to set .get_ts_info. This caused SOF_TIMESTAMPING_TX_SOFTWARE
capability to be not available.
Signed-off-by: Andreas K. Besslein <besslein.andreas@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>