batman-adv: Fix typo "reseved" -> "reserved"

checkpatch.pl complains since commit 45e417022023 ("scripts/spelling.txt:
add more spellings to spelling.txt") about an additional spelling mistake
in batman-adv:`

  CHECK: 'reseved' may be misspelled - perhaps 'reserved'?
  #232: FILE: include/uapi/linux/batadv_packet.h:232:
  + * @flags: reseved for routing relevant flags - currently always 0

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