iwlwifi: mvm: limit aggregation size in low latency to 6

This is a tradeoff between enabling better throughput for
bursty traffic and low latency. The number 6 was found to be
a good tradeoff for the Miracast use case which is the
major use case for low latency.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>