net: bgmac: store MAC address directly in netdev->dev_addr

After commit 34a5102c ("net: bgmac: allocate struct bgmac just once
& don't copy it") the mac_addr member of struct bgmac is no longer
necessary to pass the MAC address to bgmac_enet_probe(). Instead it can
directly be stored in netdev->dev_addr.

Also use eth_hw_addr_random() instead of eth_random_addr() in case a
random MAC is nedded. This will make sure netdev->addr_assign_type will
be properly set.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>