ethernet: add a helper for assigning port addresses

We have 5 drivers which offset base MAC addr by port id.
Create a helper for them.

This helper takes care of overflows, which some drivers
did not do, please complain if that's going to break
anything!

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Shannon Nelson <snelson@pensando.io>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>