powerpc/powernv: add missing of_node_put() in opal_export_attrs()

After using 'np' returned by of_find_node_by_path(), of_node_put()
need be called to decrease the refcount.

Fixes: 11fe909d ("powerpc/powernv: Add OPAL exports attributes to sysfs")
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220906141703.118192-1-zhengyongjun3@huawei.com