tools/testing/cxl: Fix root port to host bridge assignment

Mocked root-ports are meant to be round-robin assigned to host-bridges.

Fixes: 67dcdd4d ("tools/testing/cxl: Introduce a mocked-up CXL port hierarchy")
Link: https://lore.kernel.org/r/164298431629.3018233.14004377108116384485.stgit@dwillia2-desk3.amr.corp.intel.com


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