IB/mlx5: Don't return errors from poll_cq

Remove returning errors from mlx5 poll_cq function. Polling CQ
operation in kernel never fails by Mellanox HCA architecture and
respective driver design.

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