batman-adv: use ETH_P_BATMAN

The ETH_P_BATMAN ethertype is now defined kernel-wide. Use it instead
of the private BATADV_ETH_P_BATMAN define.

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