batman-adv: use batadv_compare_eth when possible

When comparing Ethernet address it is better to use the more
generic batadv_compare_eth. The latter is also optimised for
architectures having a fast unaligned access.

Signed-off-by: Antonio Quartulli <a@unstable.cc>
[sven@narfation.org: fix conflicts with current version]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>