pasemi_mac: Add support for changing mac address

Straightforward. It used to be hardcoded and impossible to override
with ifconfig.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>