IB/mlx4: Fix check of max_qp_dest_rdma in modify QP

max_qp_dest_rdma is already in natural units - no need to shift.  This
was discovered by a test that deliberately requests more outstanding
atomic operation than the device supports.

Found by Sagi Rotem at Mellanox.

Signed-off-by: Eli Cohen <eli@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>