RDMA/cm: Remove return code from add_cm_id_to_port_list

This cannot happen, all callers pass in one of the two pointers. Use
a WARN_ON guard instead.

Link: https://lore.kernel.org/r/20200506074701.9775-3-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>