net-next: dsa: make the set_addr() operation optional

Only 1 of the 3 drivers currently has a set_addr() operation. Make the
set_addr() callback optional to reduce the amount of empty stubs inside
the drivers.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>