device: fix update of the ip-iface property

Before the l3cfg rework, the ip-iface property was exported only for interfaces with an ifindex, and only in some device states.

Restore the old behavior since it is part of the API. For example, firewalld uses the property to tell which interfaces have a ifindex.

Fixes: 58287cbc ('core: rework IP configuration in NetworkManager using layer 3 configuration')