batman-adv: disable sysfs knobs when GW-mode is not implemented

Now that the GW-mode code is algorithm specific, batman-adv expects the
routing algorithm to implement some APIs to make it work.

However, such APIs are not mandatory, therefore we might have algorithms
not providing them. In this case all the sysfs knobs related to GW-mode
should be deactivated to make sure that settings injected by the user
for this feature are rejected.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>