mac80211: drop unencrypted frames if encryption is expected

This patch fixes a regression I (most likely) introduced, namely that
unencrypted frames are right now accepted even if we have a key for that
specific sender. That has very bad security implications.

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