net: mvpp2: fix use of the random mac address for PPv2.2

The MAC retrieval logic is using a variable to store an h/w stored mac
address and checks this mac against invalid ones before using it. But
the mac address is only read from h/w when using PPv2.1. So when using
PPv2.2 it defaults to its init state.

This patches fixes the logic to only check if the h/w mac is valid when
actually retrieving a mac from h/w.

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