batman-adv: make batadv_tt_save_orig_buffer() generic

This is a simple batadv_tt_save_orig_buffer() refactoring
aiming to make it more generic and avoid useless casts.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>