powerpc/83xx: Fix TSEC0 workability on MPC8313E-RDB boards

TSEC0 is connected to Vitesse 7385 5-port switch. The switch
isn't connected to any mdio bus, the link to the switch is fixed
to Full-duplex 1000 Mb/s (no pause).

This patch fixes following failure during bootup:

mdio@24520:01 not found
eth0: Could not attach to PHY
IP-Config: Failed to open eth0

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>