net: dsa: lantiq_gswip: delete irrelevant use of ds->phys_mii_mask

__of_mdiobus_register(), called right next, overwrites the phy_mask
we just configured on the bus, so this is redundant and confusing.
Delete it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>