sfc: support PTP over Ethernet

The previous patch add support for PTP over IPv6/UDP (only for 8000
series and newer) and this one add support for PTP over 802.3.

Tested: sync as master and as slave is correct with ptp4l. PTP over IPv4
and IPv6 still works fine.

Suggested-by: Edward Cree <ecree.xilinx@gmail.com>
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>