net: switchdev: extract struct switchdev_obj_*

Now that switchdev and its drivers directly use specific switchdev_obj_*
structures, move them out of the switchdev_obj union and get rif of this
outer structure.

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