mac80211: use synchronize_net() on key destroying

__ieee80211_key_destroy() calls synchronize_rcu() in
order to sync the tx path before destroying the key.

However, synching the tx path can be done with
synchronize_net() as well, which is usually faster
(the timing might be important for roaming scenarios).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>