net: hns3: add check NULL address for page pool

When page pool is not enabled, its address value is still NULL and page
pool should not be accessed, so add a check for it.

Fixes: 850bfb91

 ("net: hns3: debugfs add support dumping page pool info")
Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>