For XRC QP CQs may not exist. Check before attempting dereference.

Fixes: e126ba97 ('mlx5: Add driver for Mellanox Connect-IB adapters')
Signed-off-by: NEli Cohen <eli@mellanox.com>
Signed-off-by: NMaor Gottlieb <maorg@mellanox.com>
Reviewed-by: NYishai Hadas <yishaih@mellanox.com>
Signed-off-by: NLeon Romanovsky <leon@kernel.org>
Signed-off-by: NDoug Ledford <dledford@redhat.com>