cxl/pci: Fix NULL vs ERR_PTR confusion

cxl_pci_map_regblock() may return an ERR_PTR(), but cxl_pci_setup_regs()
is only prepared for NULL as the error case. Pick the minimal fix for
-stable backport purposes and just have cxl_pci_map_regblock() return
NULL for errors.

Fixes: f8a7e8c2 ("cxl/pci: Reserve all device regions at once")
Cc: <stable@vger.kernel.org>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163433325724.834522.17809774578178224149.stgit@dwillia2-desk3.amr.corp.intel.com


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