batman-adv: fix checkpatch warning about misspelled "cache"

commit a2d4df9b673c ("spelling.txt: add more spellings to spelling.txt")
introduced the spellcheck of "cache" for checkpatch.pl.

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