network-hotplug-rename: Improve log message when renaming a device
device: Rewrite batman-adv-port detection

When the batman-adv module is loaded, the batman_adv
subdirectory can be found in the sys tree of every network
device, so this check returned positive for every device.

The function has been rewritten to read the content of the
status file which will return the correct result.
batman-adv: Remove a now wrong assertion
bridge: Simplify the hook and make it more robust against missing ports