Disable PHY address 0 as the broadcast address, so that it can be used
as a unique (non-broadcast) address on a shared bus.

Note that this can also be configured using the B-CAST_OFF pin on
KSZ9091, but that KSZ8081 lacks this pin and is also limited to
addresses 0 and 3.

Specifically, this allows for dual KSZ8081 setups.
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>