of: Delete unnecessary check before calling "of_node_put()"

The of_node_put() function tests whether its argument is NULL and then
returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Grant Likely <grant.likely@linaro.org>