nl80211: Fix potential memory leak in nl80211_set_wowlan

Compared to cfg80211_rdev_free_wowlan in core.h,
the error goto label lacks the freeing of nd_config.
Fix that.

Signed-off-by: Ola Olsson <ola.olsson@sonymobile.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>