batman-adv: free an hard-interface before adding it

When adding a new hard interface (e.g. wlan0) to a soft interface (e.g. bat0)
and the former is already enslaved in another virtual interface (e.g. a software
bridge) batman-adv has to free it first and then continue with the adding
mechanism.

In this way the behaviour becomes consistent with what "ip link set master"
does. At the moment batman-adv enslaves the hard interface without checking for
the master device, possibly causing strange behaviours which are never wanted by
the users.

Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>