base-files: set MAC for bridge ports (devices) instead of bridge itself

base-files: set MAC for bridge ports (devices) instead of bridge itself

This restores the original config_generate behaviour. With MAC set for
bridged devices the bridge automatically gets its MAC adjusted (it picks
the lowest MAC of bridged devices).

This fixes confusing interfaces setup (bridge ports not having custom
MAC assigned).

Reported-by: Koen Vandeputte <koen.vandeputte@citymesh.com>
Fixes: c2139eef279e ("base-files: simplify setting device MAC")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit c8d8eb9d13fc6fc8b394a620c27995ee71d29c4d)