iwlwifi: tx: enable A-MSDU in low latency mode

Tests have shown that we can meet low latency KPIs with A-MSDU
enabled so enable it to achieve max TPT.

Signed-off-by: Mordechay Goodstein <mordechay.goodstein@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200425130140.e469ce6501e4.Ibdecebca830bdfbf5220693dd1f5367f7736242d@changeid