enic: Add tunable_ops support for rx_copybreak

This patch adds support for setting/getting rx_copybreak using
generic ethtool tunable.

Defines enic_get_tunable() & enic_set_tunable() to get/set rx_copybreak.
As of now, these two function supports only rx_copybreak.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>