[ETHTOOL]: Internal cleanup of ethtool_value-related handlers

Several get/set functions can be handled by a passing the ethtool_op
function pointer directly to a generic function.  This permits deletion
of a fair bit of redundant code.

Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>