batman-adv: use orig_hash_find() instead of get_orig_node() in TT code

get_orig_node() tries to retrieve an orig_node object based on a mac address
and creates it if not present. This is not the wanted behaviour in the
translation table code as we don't want to create new orig_code objects but
expect a NULL pointer if the object does not exist.

Reported-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Sven Eckelmann <sven@narfation.org>