iwlwifi: iwl_power_update_mode always hold mutex

iwl_power_update_mode expects to be called with mutex held, for example to
protect priv->vif. Only one caller currently does not do this, fix this.
Also, add a comment to iwl_power_update_mode to indicate this requirement.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>