wifi: mac80211: rework ack_frame_id handling a bit

Take one more free bit to indicate it's IDR vs. internal
usage, to be able to carve out some bits here for other
internal usage, other than IDR handling with a full ACK
SKB, that is.

Reviewed-by: Benjamin Berg <benjamin.berg@intel.com>
Reviewed-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>