luci-mod-interfaces: simplify configuring MAC address of 802.1q devices

Treat not explicitly configured, preexisting VLAN interfaces as simple
network devices when adding configuration for them, since it is more
likely that people want to set general device properties such as MAC
address instead of reconfiguring ingress/egress QoS mapping, which is
the only editable property of preexisting VLAN device config dialogs.

Ref: https://github.com/openwrt/luci/issues/5102
Signed-off-by: Jo-Philipp Wich <jo@mein.io>