mac80211: clean up RX key checks

Using the default key for "any key set" isn't
quite what we should do. It works, but with the
upcoming changes it makes life unnecessarily
complex, so do something better here and really
check for "any key".

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