batman-adv: make tt_global_add static and return bool

batadv_tt_global_add is not used anymore outside of the TT
code thanks to the TVLV implementation. It can therefore be
declared as static

Last user has been removed by 3de4e64df0f1326db7cc0ef25f5af8522850252d
("batman-adv: tvlv - convert roaming adv packet to use tvlv unicast packets")

Moreover make it return bool since its result can be either 0 or 1.

Reported-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>