tlan: Isolate external PHY when using internal PHY

When using internal 10 Mbps PHY, isolate the external PHY from MII bus.
External PHY must be kept powered up because it passes TX from tlan chip to
network.

This fixes weird link-loss problems under load with OC-2326 card at 10 Mbps.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: David S. Miller <davem@davemloft.net>