fix: Handle 0xff layers value in ZMK combos parsing

Commonly used e.g. in nodefree-config:
https://github.com/urob/zmk-nodefree-config/blob/973f717/helper.h#L65

keymap_drawer/parse/zmk.py

@@ -194,7 +194,7 @@ def _get_combos(self, dts: DeviceTree) -> list[ComboSpec]:
                 ) from err
 
             combo = {"k": parsed_key, "p": key_pos}
-            if layers := node.get_array("layers"):
+            if (layers := node.get_array("layers")) and layers[0].lower() != "0xff":
                 combo["l"] = [self.layer_names[int(layer)] for layer in layers]
 
             # see if combo had additional properties specified in the config, if so merge them in