Use the correct property names in the up and forward axis update
callbacks. Otherwise they don't work as intended and the following will
be traced to the console:
```
RNA_enum_get: WM_OT_usd_export.forward_axis not found.
RNA_enum_get: WM_OT_usd_export.up_axis not found.
```
Pull Request: https://projects.blender.org/blender/blender/pulls/124112