RDMA/cm: Use specific keyword to check define

There is a specific define keyword to check if define exists or not,
let's use it instead of open-coded variant.

Link: https://lore.kernel.org/r/20191020071559.9743-3-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>