Staging: batman-adv: suppress false warning when changing the mac address

Whenever the mac address of an batman interface is changed
check_known_mac_addr() is called to print a warning if the newly added
mac address exists an another batman interface. While looping through
the batman interface list check_known_mac_addr() only compares mac
addresses and does not make sure they belong to different interfaces,
thus always printing a warning.

Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>