net: fec: add fallback to random MAC address

If no valid MAC address could be obtained from the hardware,
fall back to a randomly generated one.

Signed-off-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>