iwlwifi: mvm: rs: use STBC regardless of power save mode

Tx STBC was used only when in CAM mode or if powersave is disabled.
Effectively this meant we never used STBC as these modes aren't
used on most platforms by default. Change that.

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