serial/sunsu: add missing of_node_put()

of_find_node_by_path() acquires a reference to the node
returned by it and that reference needs to be dropped by its caller.
This place is not doing this, so fix it.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>