net: hns3: check NULL pointer before use

This patch checks ops->set_default_reset_request whether is NULL
before using it in function hns3_slot_reset.

Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>