net: sun8i-emac: fix MDIO frequency

Commit 4f0278da ("net: sun8i-emac: Lower MDIO frequency") leads to
network failure on the OrangePi PC.

    => dhcp
    sun8i_emac_eth_start: Timeout

According to the commit message the change of the MDIO frequency is only
required for external PHYs.

Fixes: 4f0278da ("net: sun8i-emac: Lower MDIO frequency")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>