batman-adv: Group ethtool related code together

The ethtool code was spread in soft-interface.c. This makes reading the
code and working on it unnecessary complicated. Having everything in a
common place next to the other code which references it, makes it slightly
easier.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Acked-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>