iwlwifi: mvm: Do not open aggregations for null data packets

Currently we try to open an aggregation for every packet (given that one
is not already open).

This causes redundant overhead (addba/delba) for null data packets.

Do not open an aggregation for null data packets.

Signed-off-by: Matti Gottlieb <matti.gottlieb@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>