RDMA: Fix return code check in rdma_set_cq_moderation

The proper return code is "-EOPNOTSUPP" when the modify_cq() callback is
not supported, all drivers should generate this and all users should check
for it when detecting not supported functionality.

Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Acked-by: Leon Romanovsky <leonro@mellanox.com> (for mlx5)
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>