[fix] Avoided maxLength on PIN property to allow configuration variables

Author: nemesifier

netjsonconfig/backends/openwrt/schema.py

@@ -180,7 +180,6 @@
                     "pin": {
                         "type": "string",
                         "title": "PIN code",
-                        "maxLength": 4,
                         "propertyOrder": 1.2,
                     },
                     "device": {