net: pcs: xpcs: export xpcs_config_eee

There is no good reason why we need to go through:

stmmac_xpcs_config_eee
-> stmmac_do_callback
   -> mdio_xpcs_ops->config_eee
      -> xpcs_config_eee

when we can simply call xpcs_config_eee.

priv->hw->xpcs is of the type "const struct mdio_xpcs_ops *" and is used
as a placeholder/synonym for priv->plat->mdio_bus_data->has_xpcs. It is
done that way because the mdio_bus_data pointer might or might not be
populated in all stmmac instantiations.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>