wifi_statistics: Fix compatibility with Linux 3.11

The notification ptr isn't directly pointing to the net_device anymore. Thus it
has to be sent through netdev_notifier_info_to_dev to get the net_device.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>