iwlwifi: mvm: reduce the AMPDU size in low latency mode

This allows to leave a frame in the Tx Fifo which allows
the firmware to try to enter burst mode.
The end result of this is a better latency since the
firmware utilises the TxOP better.

Also limit the AMPDU size to the limit set in the ADDBA
response. This doesn't change much since the AMPDU size
was limited by the configuration of the hardware scheduler,
but here we add a software limit by the mean of the link
quality command.

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