batman-adv: don't check the source address twice

The source address has already been checked in
batadv_check_management_packet() upon packet reception and
therefore it does not need to be checked again in
ogm_process()

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