batman-adv: fix for leaked TVLV handler.

A handler for BATADV_TVLV_ROAM was being registered when the
translation-table was initialized, but not unregistered when the
translation-table was freed.  Unregister it.

Fixes: 122edaa0 ("batman-adv: tvlv - convert roaming adv packet to use tvlv unicast packets")
Reported-by:  <syzbot+d454a826e670502484b8@syzkaller.appspotmail.com>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>