ethtool: add support to set/get tx copybreak buf size via ethtool

Add support for ethtool to set/get tx copybreak buf size.

Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>