RDMA/cma: Deduplicate error flow in cma_validate_port()

Clean up to prepare for the addition of new logic.

Link: https://lore.kernel.org/r/168960674597.3007.6128252077812202526.stgit@manet.1015granger.net


Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>