batman-adv: Fix description for BATMAN_ADV_DEBUG

The debug messages of batman-adv are not printed to the kernel log at all
but can be stored (depending on the compile setting) in the tracing buffer
or the batadv specific log buffer. There is also no debug module parameter
but a batadv netdev specific log_level setting to enable/disable different
classes of debug messages at runtime.

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