ethtool: change ethtool_set_gro() to use ethtool_op_get_rx_csum

To be able to switch on GRO on a device, ethtool_set_gro() checks this
device provides a get_rx_csum() method.

Some devices dont provide this method, while they do support RX
checksumming.

This patch allows bonding to support GRO :

ethtool -K bond0 gro on

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>