net: bcmgenet: enable NETIF_F_HW_CSUM feature

The GENET hardware should be capable of generating IP checksums
using the NETIF_F_HW_CSUM feature, so switch to using that feature
instead of the depricated NETIF_F_IP_CSUM and NETIF_F_IPV6_CSUM.

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>