iwlwifi: mvm: Change location of vif_count verification for PM

Currently vif_count verification for power management enablement appear
in different places. Move these verifications to one place in
iwl_mvm_update_power_mode().

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>