net: mvpp2: parser fix PPPoE

In PPPoE add all IPv4 header option length to the parser
and adjust the L3 and L4 offset accordingly.
Currently the L4 match does not work with PPPoE and
all packets are matched as L3 IP4 OPT.

Fixes: 3f518509

 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Signed-off-by: Sven Auhagen <sven.auhagen@voleatech.de>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>