mac80211: fix debugfs default key oops

Under certain circumstances (in AP mode) the debugfs function
that is supposed to add the default key symlink can encounter
a NULL default_key pointer. This patch makes it handle that
situtation gracefully.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>