net: mvpp2: cls: Add missing NETIF_F_NTUPLE flag

Now that the mvpp2 driver supports classification offloading, we must
add the NETIF_F_NTUPLE to the features list.

Since the current code doesn't allow disabling the feature, we don't set
the flag in dev->hw_features.

Fixes: 90b509b3

 ("net: mvpp2: cls: Add Classification offload support")
Reported-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>