fjes: ethtool -w and -W support for fjes driver

This patch adds implementation of supporting
ethtool -w and -W for fjes driver.

You can enable and disable firmware debug mode by
using ethtool -W, and also retrieve firmware
activity information by using ethtool -w.

This is useful for debugging.

Signed-off-by: Taku Izumi <izumi.taku@jp.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>