RDMA/mlx5: Avoid setting redundant NULL for XRC QPs

There is no need to set NULL in recv_cq and send_cq, they are already
set to NULL by the IB/core logic.

Link: https://lore.kernel.org/r/20200427154636.381474-6-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>