net: hns3: Rename mac loopback to app loopback

In fact, our implementation of mac loopback is the implementation of app
loopback now. Current name is wrong. This patch renames mac loopback to
app loopback.

Signed-off-by: Fuyun Liang <liangfuyun1@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>