bgmac: support up to 3 cores (devices) on a bus

Broadcom buses may have more than 1 Ethernet device. This is used e.g.
to have few interfaces connected to different switch ports. So far we
saw chipsets with only 2 devices (e.g. BCM4706) but recent ones have
up to 3 (e.g. Netgear R8000 uses 3rd interface for most of switch
traffic, lower interfaces are for some kind of offloading).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>