batman-adv: Use default throughput value on cfg80211 error

commit 3f3f87325dcb3c201076c81490f4da91ad4c09fc upstream.

A wifi interface should never be handled like an ethernet devices. The
parser of the cfg80211 output must therefore skip the ethtool code when
cfg80211_get_station returned an error.

Fixes: f44a3ae9a281 ("batman-adv: refactor wifi interface detection")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Reviewed-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>