arc_emac: remove custom "max-speed" parsing code

The ARC emac driver was the only in-tree to parse a PHY device
'max-speed' property but yet failed to do it correctly because
'max-speed' is supposed to set a PHY device supported features, not the
advertising features as it was done.

Now that of_mdiobus_register() takes care of doing that, remove the
custom 'max-speed' parsing code.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>