net: bcmgenet: use CHECKSUM_COMPLETE for NETIF_F_RXCSUM

This commit updates the Rx checksum offload behavior of the driver
to use the more generic CHECKSUM_COMPLETE method that supports all
protocols over the CHECKSUM_UNNECESSARY method that only applies
to some protocols known by the hardware.

This behavior is perceived to be superior.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>