iwlwifi: rs: option to easily disable Tx MIMO

The current TLC isn't optimized for low latency and some devices
have issues with MIMO. This kind of combo creates latency issues.
Allow to temporarily disable MIMO for tests in order to solve
the latency issues without the added complexity of MIMO.

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