mac80211: MFP - Drop unprotected Action frames prior key setup

When management frame protection (IEEE 802.11w) is used, unprotected
Robust Action frames are not allowed prior to key configuration.
However, unprotected Deauthentication and Disassociation frames are
allowed at that point, but not after key configuration.

Make ieee80211_drop_unencrypted() handle the special cases for MFP by
separating the basic Data frame case from Management frame processing
and handle the Management frames only if MFP has been negotiated. In
addition, do not use sdata->drop_unencrypted for Management frames
since the decision on whether to accept the frame depends on the key
being configured.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>