net: ibmveth: convert to hw_features

A minimal conversion.

ibmveth_set_csum_offload() can be folded into ibmveth_set_features()
and adapter->rx_csum removed - left for later cleanup.

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