cxl/port: Up-level cxl_add_dport() locking requirements to the caller

In preparation for moving dport enumeration into the core, require the
port device lock to be acquired by the caller.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164367759016.324231.105551648350470000.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>