net: mvpp2: Add missing VLAN tag detection

Marvell PPv2 Header Parser sets some bits in the 'result_info' field in
each lookup iteration, to identify different packet attributes such as
DSA / VLAN tag, protocol infos, etc. This is used in further
classification stages in the controller.

It's the DSA tag detection entry that is in charge of detecting when there
is a single VLAN tag.

This commits adds the missing update of the result_info in this case.

Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>