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>
diff --git a/drivers/net/wireless/iwlwifi/iwl-debugfs.c b/drivers/net/wireless/iwlwifi/iwl-debugfs.c
index 5c8377b..d134301 100644
--- a/drivers/net/wireless/iwlwifi/iwl-debugfs.c
+++ b/drivers/net/wireless/iwlwifi/iwl-debugfs.c
@@ -815,7 +815,9 @@
 
 	priv->power_data.debug_sleep_level_override = value;
 
+	mutex_lock(&priv->mutex);
 	iwl_power_update_mode(priv, true);
+	mutex_unlock(&priv->mutex);
 
 	return count;
 }