pinctrl: mediatek: add missing of_node_put

for_each_child_of_node performs an of_node_get on each iteration, so a
return from the loop requires an of_node_put.

A simplified version of the semantic patch that fixes this problem is as
follows (http://coccinelle.lip6.fr):

// <smpl>
@@
local idexpression n;
expression e,e1;
@@

 for_each_child_of_node(e1,n) {
   ...
(
   of_node_put(n);
|
   e = n
|
   return n;
|
+  of_node_put(n);
?  return ...;
)
   ...
 }
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/mediatek/pinctrl-mtk-common.c

@@ -598,6 +598,7 @@ static int mtk_pctrl_dt_node_to_map(struct pinctrl_dev *pctldev,
 				&reserved_maps, num_maps);
 		if (ret < 0) {
 			pinctrl_utils_dt_free_map(pctldev, *map, *num_maps);
+			of_node_put(np);
 			return ret;
 		}
 	}