cxl/acpi: Convert CFMWS parsing to ACPI sub-table helpers

The cxl_acpi driver originally open-coded its table parsing since the
ACPI subtable helpers were marked __init and only used in early NUMA
initialization.  Now that those helpers have been exported for driver
usage replace the open-coded solution with the common one.

Cc: Alison Schofield <alison.schofield@intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/163553710810.2509508.14686373989517930921.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>