net: dsa: Move FDB add/del implementation inside DSA

Currently DSA uses switchdev's implementation of FDB add/del ndos. This
patch moves the implementation inside DSA in order to support the legacy
way for static FDB configuration.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>