fcoe: Use CHECKSUM_PARTIAL to indicate CRC offload

When setting up CRC offload set ip_summed to CHECKSUM_PARTIAL
instead of CHECKSUM_UNNECESSARY. This is consistent with the
definition of CHECKSUM_PARTIAL.

The only driver that seems to be advertising NETIF_F_FCOE_CRC is
ixgbe. AFICT the driver does not look at ip_summed for FCOE and
just assumes that CRC is being offloaded.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>