mac80211: restructure key selection

The "no key" case in key selection that decides
whether to drop the frame or not is impossible
to understand, restructure the code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[cavallar@lri.fr: removed blank line and restructured action frame clause]
Signed-off-by: Nicolas Cavallari <cavallar@lri.fr>