nl80211: Zero out the connection keys memory when freeing them.

The connection keys are zeroed out in all other cases except this
one. Let's fix the last one as well.

Signed-off-by: Ola Olsson <ola.olsson@sonymobile.com>
Reviewed-by: Julian Calaby <julian.calaby@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>