iwlwifi: mvm: document passing unexpected Block Ack Request frames

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: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>