[SUNGEM]: Fix MAC address setting when interface is up.

This patch implements set_mac_address for the sungem driver.  This
allows changing the mac address of the interface, even when the
interface is up.

Signed-off-by: Ruben Vandeginste <snowbender@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>