batman-adv: fixed hash functions type to uint32_t instead of int

There are two reasons for this fix:
- the result of choose_orig() and vis_choose() is an index and therefore it can't
  be negative. Hence it is correct to make the return type unsigned too.

- sizeof(int) may not be the same on ALL the architectures. Since we plan to use
  choose_orig() as DHT hash function, we need to guarantee that, given the same
  argument, the result is the same. Then it is correct to explicitly express
  the size of the return type (and the second argument). Since the expected
  length is currently 4, uint32_t is the most convenient choice.

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>