net/mlx5e: E-switch vport manager is valid for ethernet only

Currently the driver support only ethernet eswitch, and we want to
protect downstream IPoIB netdev from trying to access it in IB link.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>