net: stmmac: Create DW XPCS device with particular address

Currently the only STMMAC platform driver using the DW XPCS code is the
Intel mGBE device driver. (It can be determined by finding all the drivers
having the stmmac_mdio_bus_data::has_xpcs flag set.) At the same time the
low-level platform driver masks out the DW XPCS MDIO-address from being
auto-detected as PHY by the MDIO subsystem core. Seeing the PCS MDIO ID is
known the procedure of the DW XPCS device creation can be simplified by
dropping the loop over all the MDIO IDs. From now the DW XPCS device
descriptor will be created for the MDIO-bus address pre-defined by the
platform drivers via the stmmac_mdio_bus_data::pcs_mask field.

Note besides this shall speed up a bit the Intel mGBE probing.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>