mac80211: add option to generate CCMP IVs only for mgmt frames

Some chips can encrypt managment frames in HW, but
require generated IV in the frame. Add a key flag
that allows us to achieve this.

Signed-off-by: Marek Kwaczynski <marek.kwaczynski@tieto.com>
[use BIT(0) to fill that spot, fix indentation]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>