luci-base: reintroduce accidentally lost isCreateable() function

Fixes: FS#2976
Fixes: 711f75927 ("luci-base: harmonize JS class naming and requesting")
Ref: https://bugs.openwrt.org/index.php?do=details&task_id=2976
Signed-off-by: Jo-Philipp Wich <jo@mein.io>

modules/luci-base/htdocs/luci-static/resources/network.js

@@ -2305,6 +2305,24 @@ Protocol = baseclass.extend(/** @lends LuCI.network.Protocol.prototype */ {
 		return null;
 	},
 
+	/**
+	 * Check function for the protocol handler if a new interface is createable.
+	 *
+	 * This function should be overwritten by protocol specific subclasses.
+	 *
+	 * @abstract
+	 *
+	 * @param {string} ifname
+	 * The name of the interface to be created.
+	 *
+	 * @returns {Promise<void>}
+	 * Returns a promise resolving if new interface is createable, else
+	 * rejects with an error message string.
+	 */
+	isCreateable: function(ifname) {
+		return Promise.resolve(null);
+	},
+
 	/**
 	 * Checks whether the protocol functionality is installed.
 	 *