batman-adv: use the proper header len when checking the TTVN

Unicast packet might be of type either UNICAST or
UNICAST4ADDR.
In the two cases the header size is different, but the
mechanism checking the TTVN field was assuming it to be
always of the same type (UNICAST), so failing to access the
inner Ethernet header in case of UNICAST4ADDR.

Fix this by passing the real header length as argument.

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