RDMA/nes: Change WQ overflow return code

Change the nes driver to return -ENOMEM on SQ/RQ overflow to match the
return code of other RDMA HW drivers (e.g cxgb3, ehca, mlx4, mthca).

Signed-off-by: Or Gerlitz <ogerlitz@voltaire.com>
Acked-by: Chien Tung <chien.tin.tung@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>