batman-adv: use ETH_ALEN instead of hardcoded numeric constants

In packet.h the numeric constant 6 is used instead of the more portable ETH_ALEN
define. This patch substitute any hardcoded value with such define.

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