sfc: support PTP over IPv6/UDP

commit bd4a2697 ("sfc: use hardware tx timestamps for more than
PTP") added support for hardware timestamping on TX for cards of the
8000 series and newer, in an effort to provide support for other
transports other than IPv4/UDP.

However, timestamping was still not working on RX for these other
transports. This patch add support for PTP over IPv6/UDP.

Tested: sync as master and as slave is correct using ptp4l from linuxptp
package, both with IPv4 and IPv6.

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>