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

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>
(cherry picked from commit 0e459668c5b3b158991803204f628b1b7dce9034)