net/mlx5e: NIC netdev init flow cleanup

Remove redundant call to unregister vport representor in mlx5e_add error
flow.

Hide the representor priv and eswitch internal structures from en_main.c
as preparation step for downstream patches which would allow building
the driver without support for  representors and eswitch.

Fixes: 6f08a22c ("net/mlx5e: Register/unregister vport representors on interface attach/detach")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>