batman-adv: only drop packets of known wifi clients

batman-adv: only drop packets of known wifi clients

bug introduced with 2d858257429c8df9a6c030c1501b26f45279a672

If the source or destination mac address of an ethernet packet
could not be found in the translation table the packet was
dropped if AP isolation was turned on. This behavior would
make it impossible to send broadcast packets over the mesh as
the broadcast address will never enter the translation table.

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