batman-adv: pass a 16bit long flag argument to tt_global_add()

it may be the case that we want to store some local TT client flags in a global
entry, therefore the tt_global_add needs to get a proper argument for this

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>