IB/mlx4: Support modify_qp for RoCE v2

In order to support modify_qp for RoCE v2, we need to set
the gid_type in the QP context.

Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>