8139cp: allow to set mac address on running device

So far there was not a chance to set a mac address on running 8139cp device.
This is for example needed when you want to use this NIC as a bonding slave in
bonding device in mode balance-alb. This simple patch allows it.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>