net: hns3: add priv flags support to switch limit promisc mode

Currently, the tx unicast promisc is always enabled when promisc
mode on. If tx unicast promisc on, a function will receive all
unicast packet from other functions belong to the same port.
Add a ethtool private flag to control whether enable tx
unicast promisc. Then the function is able to filter the
unknown unicast packets from other function.

Signed-off-by: Jian Shen <shenjian15@huawei.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>