net: mvpp2: cls: Add missing MAC_DA field extraction

PPv2's classifier supports extracting the MAC Destination Address from the
L2 header to perform RSS and flow steering. Add the missing case when
setting the Header Extracted Key fields in the flow table.

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