mac80211: fix debugfs default key links

Due to the way the default key links are created,
it happens that a link is left dangling:
 * both unicast/multicast links are created
 * unicast link is destroyed, and the links
   are updated
 * during this update, adding the multicast
   link again fails because it is present,
   destroying the debugfs pointer
 * removing the multicast link won't work as
   the pointer has been destroyed

Fix this by always removing the links and then
re-creating them if needed.

Reported-by: Marek Lindner <lindner_marek@yahoo.de>
Reported-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>