batman-adv: Fix checkpatch warning about 'unsigned' type

checkpatch.pl warns about the use of 'unsigned' as a short form for
'unsigned int'.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <a@unstable.cc>