mac80211: make sure hw_key exists before checking its flags

Fixes a bug introduced in:

commit 077a9154
Author: Arik Nemtsov <arik@wizery.com>
Date:   Sun Oct 23 08:21:41 2011 +0200

Reported-by: Arend van Spriel <arend@broadcom.com>
Reported-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>