netlink-specs: add rx-push to ethtool family

Commit 5b4e9a7a ("net: ethtool: extend ringparam set/get APIs for rx_push")
added a new attr for configuring rx-push, right after tx-push.
Add it to the spec, the ring param operation is covered by
the otherwise sparse ethtool spec.

Reviewed-by: Shannon Nelson <shannon.nelson@amd.com>
Link: https://lore.kernel.org/r/20230214043246.230518-1-kuba@kernel.org


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