batman-adv: Allow to use BATMAN_ADV_DEBUG without BATMAN_ADV_DEBUGFS

The BATMAN_ADV_DEBUGFS portion of batman-adv is marked as deprecated. Thus
all required functionality should be available without it. The debug log
was already modified to also output via the kernel tracing function but
still retained its BATMAN_ADV_DEBUGFS functionality.

Separate the entry point for the debug log from the debugfs portions to
make it possible to build with BATMAN_ADV_DEBUG and without
BATMAN_ADV_DEBUGFS.

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