[PATCH] ethtool: allow const ethtool_ops

The ethtool_ops structure is immutable, it expected to be setup
by the driver and is never changed. This patch allows drivers to
declare there ethtool_ops structure read-only.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>