base-files: fix configuration generation of network if "bridge" exists

After the commit 43fc7206
("base-files: generate "device UCI type section for bridge"), the wrong
network configuration is generated for the devices that already have the
bridge device section for VLAN, such as the devices in realtek target.

As a result, the bridge device by additional "device" section is
specified to the "ports" option in the "bridge-vlan" section and netifd
shuts down the switch and the ethernet when the network service started.

Fixes: 43fc7206 ("base-files: generate "device" UCI type section for bridge")
Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
[rmilecki: use $ports for generate_bridge_vlan argument]
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit 8cc4e87a)