base-files: generate bridge device sections with br- name prefix

Missing br- prefix could result in name conflict between DSA port
interface and bridge interface. Some devices with just one LAN port use
"lan" interface name for DSA port. Trying to create bridge with the same
"lan" name was failing.

Reported-by: David Bauer <mail@david-bauer.net>
Fixes: 43fc720657c6 ("base-files: generate "device" UCI type section for bridge")
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>