batman-adv: Disable CONFIG_BATMAN_ADV_SYSFS by default

The sysfs support in batman-adv is deprecated since a while and will be
removed completely next year.

All tools which were known to the batman-adv development team are
supporting the batman-adv netlink interface since a while. Thus
disabling CONFIG_BATMAN_ADV_SYSFS 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>