ixp4xx_eth: Remove setting of RX software timestamp

ixp4xx_eth: Remove setting of RX software timestamp

The responsibility for reporting of RX software timestamp has moved to
the core layer (see __ethtool_get_ts_info()), remove usage from the
device drivers.

Reviewed-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Gal Pressman <gal@nvidia.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patch.msgid.link/20240906144632.404651-16-gal@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>