IB/mlx5: Unify e-switch representors load approach between uplink and VFs

When in switchdev mode and the add function is called by the core
level driver, make sure we only register the callbacks, but don't
create the mlx5 IB device or initialize anything. With this change
all the IB devices in switchdev mode are created only once the load
callback is invoked by the e-switch core sub-module. This follows
the design paradigm under which the all the Eth representors must
be loaded before any of IB reprs is loaded.

Signed-off-by: Mark Bloch <markb@mellanox.com>
Acked-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>