net/mlx5e: Move ethernet representors data into separate struct

Ethernet representors have a need to store data which is applicable
only for them. Create a priv void pointer in struct mlx5_eswitch_rep
and move mlx5e to store the relevant data there. As part of this change
we also initialize rep_if in mlx5e_rep_register_vf_vports() as otherwise the
E-Switch code will copy a priv value which is garbage.

We also rename mlx5_eswitch_get_uplink_netdev() to
mlx5_eswitch_get_uplink_priv() and make it return void *.
This way E-Switch code doesn't need to deal with net devices and
we leave the task of getting it to mlx5e.

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