core/pci: pci_slot_add_loc use null-terminated strings

Use null-terminated strings consistently, making the maximum string
length in all cases the same, and avoiding dt_add_property_nstr.

This avoids the following warning that appears after adding more
checking to string ops:

  core/pci-slot.c: In function ‘pci_slot_add_loc’:
  skiboot/libc/include/string.h:19:17: warning: ‘__builtin_strncpy’
    specified bound 80 equals destination size [-Wstringop-truncation]
   #define strncpy __builtin_strncpy
  core/pci-slot.c:244:3: note: in expansion of macro ‘strncpy’
     strncpy(loc_code, label, sizeof(loc_code));
     ^~~~~~~

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Stewart Smith <stewart@linux.ibm.com>

core/pci-slot.c

@@ -241,10 +241,10 @@ void pci_slot_add_loc(struct pci_slot *slot,
 		snprintf(loc_code, sizeof(loc_code), "%s-%s",
 			phb->base_loc_code, label);
 	} else {
-		strncpy(loc_code, label, sizeof(loc_code));
+		strncpy(loc_code, label, sizeof(loc_code) - 1);
+		loc_code[LOC_CODE_SIZE - 1] = '\0';
 	}
 
 	dt_add_property_string(np, "ibm,slot-label", label);
-	dt_add_property_nstr(np, "ibm,slot-location-code", loc_code,
-				sizeof(loc_code));
+	dt_add_property_string(np, "ibm,slot-location-code", loc_code);
 }