batman-adv: only drop packets of known wifi clients

batman-adv: only drop packets of known wifi clients

bug introduced with 59b699cdee039d75915c354da06937102d1f9a84

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>