net: hns3: add existence checking before adding unicast mac address

It's not allowed to add two same unicast mac address entries to the
mac_vlan table. When modify the uc mac address of a VF device to the
same value with the PF device's, the PF device will lose its entry of
the mac_vlan table.

Lookup the mac address in the mac_vlan table, and add it if the entry
is inexistent.

Fixes: 46a3df9f ("net: hns3: Add HNS3 Acceleration Engine & Compatibility Layer Support")
Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>