batman-adv: Use ethtool helper to get link status

The ethtool_ops of batman-adv never contained more than a stub for the
get_link function pointer. It was always returning that a link exists even
when the devices was not yet up and therefore nothing resampling a link
could have been available.

Instead use the ethtool helper which returns the current carrier state.

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