The old way of defining bridge (L2) as part of interface (L3) is
deprecated. Don't support *adding* interfaces like that.
Support for *editing* legacy bridges is kept for now for compatibility
with existing legacy setups.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit b7f3cf66cadf6753153bc9d1feac1eca0c7e37f0)