net: ethernet: ax88796: support generating a random mac address

Instead of falling back to 00:00:00:00:00:00 generate a random address
if none is provided via platform data or from the device's register
space.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>