cfg80211: clear wep keys after disconnection

When a low level driver calls cfg80211_disconnected(), wep keys are
not cleared. As a result, following connection requests will fail
since cfg80211 internal state shows a connection is still in progress.

Fix this by clearing the wep keys when disconnecting.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>