adi: adapt to eth_hw_addr_random() and changes in arch/blackfin

Adapt adi ethernet driver to changes in bfin_get_ether_addr()
from arch/blackfin. bfin_get_ether_addr() returns now a state.
Set a random mac address via new eth_hw_addr_random() in case
the return value is not 0.

Reset the state to NET_ADDR_PERM as soon as the MAC get
changed via .ndo_set_mac_address.

v2: change the logic to reduce unneeded checks

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Signed-off-by: David S. Miller <davem@davemloft.net>