net: benet: convert to hw_features

Simple conversion.

This also fixes a bug in TX checksum toggling --- driver was changing
NETIF_F_HW_CSUM instead of NETIF_F_IP_CSUM+NETIF_F_IPV6_CSUM.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>