net: mvpp2: prs: fix PPPoE with ipv6 packet parse

Current PPPoE+IPv6 entry is jumping to 'next-hdr'
field and not to 'DIP' field as done for IPv4.

Fixes: 3f518509 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Reported-by: Liron Himi <lironh@marvell.com>
Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Link: https://lore.kernel.org/r/1608230266-22111-1-git-send-email-stefanc@marvell.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>