The authentication frame has a fixied size of 30 bytes
(including header, algo num, trans seq num, and status)
followed by a variable challenge text.
Allocate using exact size, instead of over-allocation
by sizeof(ieee80211_mgmt).
Signed-off-by: NFred Zhou <fred.zy@gmail.com>
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>