IB/mlx5: Fix represent correct netdevice in dual port RoCE

In commit bcf87f1d ("IB/mlx5: Listen to netdev register/unresiter events in switchdev mode")
incorrectly mapped primary device's netdevice to 2nd port netdevice.
It always represented primary port's netdevice for 2nd port netdevice
when ib representors were not used.

This results into failing to process CM request arriving on 2nd port due
to incorrect mapping of netdevice.

This fix corrects it by considering the right mdev.

Cc: <stable@vger.kernel.org> # 4.16
Fixes: bcf87f1d ("IB/mlx5: Listen to netdev register/unresiter events in switchdev mode")
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>