net: dsa: dsa_loop: remove .set_addr

The .set_addr function does nothing, remove the dsa_loop implementation
before getting rid of it completely in DSA.

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