ethtool: RX software timestamp for all

All devices support SOF_TIMESTAMPING_RX_SOFTWARE by virtue of
net_timestamp_check() being called in the device independent code.

Move the responsibility of reporting SOF_TIMESTAMPING_RX_SOFTWARE and
SOF_TIMESTAMPING_SOFTWARE, and setting PHC index to -1 to the core.
Device drivers no longer need to use them.

Suggested-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Link: https://lore.kernel.org/netdev/661550e348224_23a2b2294f7@willemb.c.googlers.com.notmuch/


Co-developed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Marc Kleine-Budde <mkl@pengutronix.de>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240901112803.212753-2-gal@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>