net: dsa: notify drivers of MAC address changes on user ports

In some cases, drivers may need to veto the changing of a MAC address on
a user port. Such is the case with KSZ9477 when it offloads a HSR device,
because it programs the MAC address of multiple ports to a shared
hardware register. Those ports need to have equal MAC addresses for the
lifetime of the HSR offload.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>