sh_eth: use random MAC address if no valid one supplied

On Renesas R-Car based development boards, although a MAC address is printed on
all the Ethernet port labels, U-Boot  doesn't write a valid MAC address  to the
Ether MAHR/MALR registers (there's no storage provided for the Ether MAC address
either), so we have to resort to using a random MAC address...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>