IB/mlx5: Support IB device's callback for getting the link layer

Make the existing mlx5_ib_port_link_layer() signature match
the ib device callback signature (add port_num parameter).
Refactor it to use a sub function so that the link layer could
be queried also before the ibdev is created.

Signed-off-by: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>