net: switchdev: change fdb addr for a byte array

The address in the switchdev_obj_fdb structure is currently represented
as a pointer. Replacing it for a 6-byte array allows switchdev to carry
addresses directly read from hardware registers, not stored by the
switch chip driver (as in Rocker).

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