net: pcs: xpcs: export xpcs_validate

Calling a function pointer with a single implementation through
struct mdio_xpcs_ops is clunky, and the stmmac_do_callback system forces
this to return int, even though it always returns zero.

Simply remove the "validate" function pointer from struct mdio_xpcs_ops
and replace it with an exported xpcs_validate symbol which is called
directly by stmmac.

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>