Generate the proactive PREP element in Proactive PREQ mode as
defined in Sec. 13.10.10.3 (Case D) of IEEE Std. 802.11-2012.
Signed-off-by: NChun-Yeow Yeoh <yeohchunyeow@gmail.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>