luci-mod-network: interfaces.js: derive status from layer 3 device

Align the status column device selection logic with the approach used for
rendering the interface status boxes; first try to obtain the active
layer3 device as reported by ubus, only then fall back to the protocol
specific device hint which might or might not point to an existing device.

Fixes: #3076
Signed-off-by: Jo-Philipp Wich <jo@mein.io>