batman-adv: use eth_hdr() when it makes sense

Instead of casting the result of skb_mac_header() to
"struct ethhdr *" every time, the eth_hdr inline function
can be use to beautify the code and improve its readability.

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