batman-adv: Fix genl notification for throughput_override

The throughput_override sysfs file is not below the meshif but below a
hardif. The kobj has therefore not a pointer which can be used to find the
batadv_priv data. The pointer stored in the hardif object must be used
instead to find the correct meshif private data.

Fixes: 7e6f461e ("batman-adv: Trigger genl notification on sysfs config change")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>