arch/blackfin: don't generate random mac in bfin_get_ether_addr()

Changed bfin_get_ether_addr() to return a state and to
set no random mac address if the board don't provide one.

Let the caller of bfin_get_ether_addr() set a random mac
address if the return value is not 0.

v2: don't set random mac in bfin_get_ether_addr()

Signed-off-by: Danny Kukawka <danny.kukawka@bisect.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Bob Liu <lliubbo@gmail.com>