net: stmmac: fix NPD with phylink_set_pcs if there is no MDIO bus

priv->plat->mdio_bus_data is optional, some platforms may not set it,
however we proceed to look straight at priv->plat->mdio_bus_data->has_xpcs.

Since the xpcs is instantiated based on the has_xpcs property, we can
avoid looking at the priv->plat->mdio_bus_data structure altogether and
just check for the presence of the xpcs pointer.

Fixes: 11059740

 ("net: pcs: xpcs: convert to phylink_pcs_ops")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>