mlx4, mlx5, mthca: Expose max_sge_rd correctly

Applications must not assume that max_sge and max_sge_rd are the same,
Hence expose max_sge_rd correctly as well.

Reported-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>