r8169: support IPv6

Support the IPv6 hw checksum for RTL8111C and later chips. Note
that the hw has the limitation for the transport offset. The
checksum must be calculated by sw, when the transport offset is
out of the range which the hw accepts.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>