iwlwifi: mvm: remove usage of power_save module parameter

Make power management in MVM driver enabled by default and
remove using the power_save module parameter. Rely only on
the power_scheme parameter to decide if power management
should be used.

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