net/mlx5: Move helper to eswitch layer

To use port number to port index conversion at eswitch level, move it to
eswitch header.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>