atl1c: improve performance by avoiding unnecessary pcie writes on xmit

The kernel has xmit_more facility that hints the networking driver xmit
path about whether more packets are coming soon. This information can be
used to avoid unnecessary expensive PCIe transaction per tx packet.

Max TX pps on Mikrotik 10/25G NIC in a Threadripper 3960X system
improved from 1150Kpps to 1700Kpps.

Testing L2 forwarding on AR8151 hardware did not reveal a measurable
increase in latency.

Signed-off-by: Gatis Peisenieks <gatis@mikrotik.com>
Signed-off-by: David S. Miller <davem@davemloft.net>