iwlwifi: mvm: add function to update only ps

Add a new iwl_mvm_power_update_ps() function that allows only ps to be
updated according to changes in the vifs.  This allows us to disable
ps only without affecting the pm values of the vifs (and to avoid
sending unnecessary MAC_PM_POWER_TABLE commands to the firmware).

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