batman-adv: Limit number of aggregated packets directly

batman-adv: Limit number of aggregated packets directly

The currently selected size in BATADV_MAX_AGGREGATION_BYTES (512) is chosen
such that the number of possible aggregated packets is lower than 32. This
number must be limited so that the type of
batadv_forw_packet->direct_link_flags has enough bits to represent each
packet (with the size of at least 24 bytes).

This requirement is better implemented in code instead of having it inside
a comment.

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