net: dsa: remove .set_addr

Now that there is no user for the .set_addr function, remove it from
DSA. If a switch supports this feature (like mv88e6xxx), the
implementation can be done in the driver setup.

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