The current loopback mode is to add 0x1F to the SMAC address
as the DMAC address and enable the promiscuous mode.
However, if the VF address is the same as the DMAC address,
the loopback test fails.

Loopback can be enabled in three places: SSU, MAC, and serdes.
By default, SSU loopback is enabled, so if the SMAC and the DMAC
are the same, the packets are looped back in the SSU. If SSU loopback
is disabled, packets can reach MAC even if SMAC is the same as DMAC.

Therefore, this patch disables the SSU loopback before the loopback
test. In this way, the SMAC and DMAC can be the same, and the
promiscuous mode does not need to be enabled. And this is not
valid in version 0x20.

This patch also uses a macro to replace 0x1F.

Fixes: c39c4d98 ("net: hns3: Add mac loopback selftest support in hns3 driver")
Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Reviewed-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>