iwlwifi: advertise maximal MPDU length when Rx MQ is supported

The new hardware that supports multiple queue also
de-aggregates A-MSDUs. This means that we can advertise
the maximal size of A-MSDUs regardless of the receive
buffer's size.
In order to be able to forcefully use a lower A-MSDU size,
add a default value for the module parameter. Pre-9000
will have a default of 4K, and 9000 will have 12K.
Setting the amsdu_size module parameter to 4K will limit
the A-MSDU on 9000 as well.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>