When we get an unexpected Block Ack Request frame, the BAID from
the hardware will be invalid, and we'll pass it to mac80211 for
further handling (sending a delBA action frame.)

Add a comment explaining that, in case anyone looks in the future.
Signed-off-by: NJohannes Berg <johannes.berg@intel.com>
Signed-off-by: NLuca Coelho <luciano.coelho@intel.com>