libnvdimm, pfn: fix restoring memmap location

This path was missed when turning on the memmap in pmem support.  Permit
'pmem' as a valid location for the map.
Reported-by: NJeff Moyer <jmoyer@redhat.com>
Signed-off-by: NDan Williams <dan.j.williams@intel.com>

drivers/nvdimm/pfn_devs.c

@@ -301,10 +301,8 @@ int nd_pfn_validate(struct nd_pfn *nd_pfn)
 
 	switch (le32_to_cpu(pfn_sb->mode)) {
 	case PFN_MODE_RAM:
-		break;
 	case PFN_MODE_PMEM:
-		/* TODO: allocate from PMEM support */
-		return -ENOTTY;
+		break;
 	default:
 		return -ENXIO;
 	}