batman-adv: Fix spelling error in term buffer

checkpatch warns about a typo in the word bufFer which was introduced in
commit 2191c1bcbc64 ("batman-adv: kernel doc for types.h").

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