RDMA/mlx4: Properly annotate link layer variable

The rdma_port_get_link_layer() returns enum rdma_link_layer as
a return value, hence it is better to store the return value in
specially annotated variable and not in int.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>