net/sfc: allow PPPoE flow pattern items

net/sfc: allow PPPoE flow pattern items

These items allow the user to avoid having to set the EtherType field in an
ETH item to match PPPoE traffic. Using a PPPoED (PPPoE discovery) or PPPoES
(PPPoE session) item will lead to EtherType filter being set up with
a corresponding value. If an ETH item provides its own EtherType value,
it will be checked for correctness.

Matching on PPPoE fields is not supported.

Signed-off-by: Viacheslav Galaktionov <viacheslav.galaktionov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>