batman-adv: Disable CONFIG_BATMAN_ADV_DEBUGFS by default

All tools which were known to the batman-adv development team are
supporting the batman-adv netlink interface since a while. Also debugfs is
not supported for batman-adv interfaces in any non-default netns. Thus
disabling CONFIG_BATMAN_ADV_DEBUGFS by default should not cause problems on
most systems. It is still possible to enable it in case it is still
required in a specific setup.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>