get mac address interface - check that before creating notification

This is to address problem that allowing dev-detect and then creating guest wifi creates notification about new device.
The MAC address of br-guest is created randomly, so this cannot be solved by initialization of known MACs at install time.
The detection thus has to be dynamic and I seen this as the easiest way how to solve it.

We are planning to move to ip neigh later anyway, so this won't be needed then.