ethtool: Add generic options for tunables

This patch adds new ethtool cmd, ETHTOOL_GTUNABLE & ETHTOOL_STUNABLE for getting
tunable values from driver.

Add get_tunable and set_tunable to ethtool_ops. Driver implements these
functions for getting/setting tunable value.

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

diff --cc include/linux/ethtool.h
Simple merge

diff --cc include/uapi/linux/ethtool.h
Simple merge

diff --cc net/core/ethtool.c
Simple merge