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>

drivers/net/wireless/iwlwifi/iwl-debugfs.c

@@ -815,7 +815,9 @@ static ssize_t iwl_dbgfs_sleep_level_override_write(struct file *file,
 
 	priv->power_data.debug_sleep_level_override = value;
 
+	mutex_lock(&priv->mutex);
 	iwl_power_update_mode(priv, true);
+	mutex_unlock(&priv->mutex);
 
 	return count;
 }

drivers/net/wireless/iwlwifi/iwl-power.c

@@ -303,7 +303,7 @@ static int iwl_set_power(struct iwl_priv *priv, struct iwl_powertable_cmd *cmd)
 				sizeof(struct iwl_powertable_cmd), cmd);
 }
 
-
+/* priv->mutex must be held */
 int iwl_power_update_mode(struct iwl_priv *priv, bool force)
 {
 	int ret = 0;