net: dsa: mv88e6xxx: factorize MAC address setting

Some switch models have a dedicated register for Switch MAC/WoF/WoL.
This register, when present, is used to indirectly set the switch MAC
address, instead of a direct write to 3 global registers.

Identify this feature and share a common mv88e6xxx_set_addr function.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>