net: hns3: add a check to pointer in error_detected and slot_reset

If we add a VF without loading hclgevf.ko and then there is a RAS error
occurs, PCIe AER will call error_detected and slot_reset of all functions,
and will get a NULL pointer when we check ad_dev->ops->handle_hw_ras_error.
This will cause a call trace and failures on handling of follow-up RAS
errors.

This patch check ae_dev and ad_dev->ops at first to solve above issues.

Signed-off-by: Weihang Li <liweihang@hisilicon.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>