s390/qeth: support configurable RX copybreak

Implement the ethtool hooks for the ETHTOOL_RX_COPYBREAK tunable.

The copybreak is stored into netdev_priv, so that we automatically go
back to the default value if the netdev is re-allocated.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>