selftests: drv-net: add helper to wait for HW stats to sync

Some devices DMA stats to the host periodically. Add a helper
which can wait for that to happen, based on frequency reported
by the driver in ethtool.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20240626012456.2326192-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>