iwlwifi: 22000 devices: restrict to HT A-MPDU size

Our current HE devices don't support BlockAck with the large bitmap,
so can't do TX aggregation with 256 frames. Restrict to the lower HT
size.

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