batman-adv: h_vlan_encapsulated_proto access refactoring

In case of a VLAN tagged frame the ethhdr pointer is
moved forward by 4 bytes so that the offset of h_proto
in struct ethhdr matches the real
h_vlan_encapsulated_proto address in the skb. While this
trickery is correct it makes the code harder to understand
and may lead to bugs in case of re-use of ethhdr for other
purposes.

This patch introduces a proto variable to make things
cleaner and easier to understand.

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