batman-adv: Mark debugfs functionality as deprecated

CONFIG_BATMAN_ADV_DEBUGFS is disabled by default because debugfs is not
supported for batman-adv interfaces in any non-default netns. Any remaining
users of this interface should still be informed about the deprecation and
the generic netlink alternative.

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