net: hns3: add existence check when remove old uc mac address

When driver is in initial state, the mac_vlan table table is empty.
So the delete operation for mac address must fail. Existence check
is needed here. Otherwise, the error message will make user confused.

Fixes: 46a3df9f

 ("net: hns3: Add HNS3 Acceleration Engine & Compatibility Layer Support")
Signed-off-by: Fuyun Liang <liangfuyun1@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>