bnxt_en: use eth_hw_addr_random()

Use eth_hw_addr_random() to set a random MAC address in order to make
sure bp->dev->addr_assign_type will be properly set to NET_ADDR_RANDOM.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>