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.com


Signed-off-by: Rob Herring <robh@kernel.org>