net: mvpp2: fallback using h/w and random mac if the dt one isn't valid

When using a mac address described in the device tree, a check is made
to see if it is valid. When it's not, no fallback is defined. This
patches tries to get the mac address from h/w (or use a random one if
the h/w one isn't valid) when the dt mac address isn't valid.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>