net: hns3: Change the dst mac addr of loopback packet

Currently, the dst mac addr of loopback packet is the same as
the host' mac addr, the SSU component may loop back the packet
to host before the packet reaches mac or serdes, which will defect
the purpose of mac or serdes selftest.

This patch changes it by adding 0x1f to the last byte of dst mac
addr.

Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>