batman-adv: Convert batadv_dat_addr_t to proper type

The #define for batadv_dat_addr_t is doing nothing else than giving u16 a
new typename. But C already has the special keyword "typedef" which is also
better supported by kernel-doc.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>