cxl/region: Fix region setup/teardown for RCDs

RCDs (CXL memory devices that link train without VH capability and show
up as root complex integrated endpoints), hide the presence of the link
between the endpoint and the host-bridge. The CXL region setup/teardown
paths assume that a link hop is present and go looking for at least one
'struct cxl_port' instance between the CXL root port-object and an
endpoint port-object leading to crashes of the form:

    BUG: kernel NULL pointer dereference, address: 0000000000000008
    [..]
    RIP: 0010:cxl_region_setup_targets+0x3e9/0xae0 [cxl_core]
    [..]
    Call Trace:
     <TASK>
     cxl_region_attach+0x46c/0x7a0 [cxl_core]
     cxl_create_region+0x20b/0x270 [cxl_core]
     cxl_mock_mem_probe+0x641/0x800 [cxl_mock_mem]
     platform_probe+0x5b/0xb0

Detect RCDs explicitly and skip walking the non-existent port hierarchy
between root and endpoint in that case.

While this has been a problem since:

commit 0a19bfc8 ("cxl/port: Add RCD endpoint port enumeration")

...it becomes a more reliable crash scenario with the new autodiscovery
implementation.

Fixes: a32320b7 ("cxl/region: Add region autodiscovery")
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/168002858268.50647.728091521032131326.stgit@dwillia2-xfh.jf.intel.com


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