r8169: add check for PHY_MDIO_CHG to rtl_nic_fw_data_ok

Only values 0 and 1 are currently defined as parameters for
PHY_MDIO_CHG. Instead of silently ignoring unknown values and
misinterpreting the firmware code let's explicitly check.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>