mac80211: Add MIC space only for TX key option

Add a key flag to indicates that the device only needs
MIC space and not a real MIC.
In such cases, keep the MIC zeroed for ease of debug.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>