iwlwifi: mvm: support A-MSDU in A-MPDU

Since A-MPDU deaggregation is done in hardware, and A-MSDU deaggregation
is done in software, there's no reason not to support A-MSDU in A-MPDU;
set the flag to support it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>