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: NIra Weiny <ira.weiny@intel.com>
Reviewed-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163433325724.834522.17809774578178224149.stgit@dwillia2-desk3.amr.corp.intel.comSigned-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/cxl/pci.c

@@ -317,7 +317,7 @@ static void __iomem *cxl_pci_map_regblock(struct cxl_mem *cxlm,
 	if (pci_resource_len(pdev, bar) < offset) {
 		dev_err(dev, "BAR%d: %pr: too small (offset: %#llx)\n", bar,
 			&pdev->resource[bar], (unsigned long long)offset);
-		return IOMEM_ERR_PTR(-ENXIO);
+		return NULL;
 	}
 
 	addr = pci_iomap(pdev, bar, 0);