batman-adv: Fix kerneldoc over 80 column lines

Kerneldoc required single line documentation in the past (before 2009).
Therefore, the 80 columns limit per line check of checkpatch was disabled
for kerneldoc. But kerneldoc is not excluded anymore from it and checkpatch
now enabled the check again.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>