net: hns3: add support for TX push mode

For the device that supports the TX push capability, the BD can
be directly copied to the device memory. However, due to hardware
restrictions, the push mode can be used only when there are no
more than two BDs, otherwise, the doorbell mode based on device
memory is used.

Signed-off-by: Yufeng Mo <moyufeng@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>