net: dsa: Remove prepare phase for FDB

The prepare phase for FDB add is unneeded because most of DSA devices
can have failures during bus transactions (SPI, I2C, etc.), thus, the
prepare phase cannot guarantee success of the commit stage.

The support for learning FDB through notification chain, which will be
introduced in the following patches, will provide the ability to notify
back the bridge about successful offload.

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>