bnx2x: fix rx checksum validation for IPv6

Commit d6cb3e41 "bnx2x: fix checksum validation" caused a performance
regression for IPv6. Rx checksum offload does not work. IPv6 packets
are passed to the stack with CHECKSUM_NONE.

The hardware obviously cannot perform IP checksum validation for IPv6,
because there is no checksum in the IPv6 header. This should not prevent
us from setting CHECKSUM_UNNECESSARY.

Tested on BCM57711.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Acked-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>