net: etherdevice: add address inherit helper

Some etherdevices inherit their address from a parent or
master device. The addr_assign_type should be updated along
with the address in these cases.  Adding a helper function
to simplify this.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>