cxl/region: Fix IS_ERR() vs NULL check

The nvdimm_pmem_region_create() function returns NULL on error.  It does
not return error pointers.

Fixes: 04ad63f0

 ("cxl/region: Introduce cxl_pmem_region objects")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/Yuo65lq2WtfdGJ0X@kili
Signed-off-by: Dan Williams <dan.j.williams@intel.com>