of/unittest: Add of_node_put() before return

Fix following coccicheck warning:
./drivers/of/unittest.c:3091:1-23: WARNING: Function
for_each_child_of_node should have of_node_put() before return

Early exits from for_each_child_of_node should decrement the
node reference counter.
Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Link: https://lore.kernel.org/r/20211015082658.19005-1-wanjiabing@vivo.comSigned-off-by: Rob Herring <robh@kernel.org>

drivers/of/unittest.c

@@ -3094,6 +3094,8 @@ static __init void of_unittest_overlay_high_level(void)
 			if (!strcmp(np->full_name, base_child->full_name)) {
 				unittest(0, "illegal node name in overlay_base %pOFn",
 					 np);
+				of_node_put(np);
+				of_node_put(base_child);
 				return;
 			}
 		}