editor: fix missing user/password when loading wifi connection to edit

When you opened an existing wifi in nm-connection-editor, the username
and password field were unset for WPA & WPA2 Enterprise.

(This regression was introduced by commit c9476e9c)

https://bugzilla.redhat.com/show_bug.cgi?id=1000564



Signed-off-by: Thomas Haller <thaller@redhat.com>