ice: Add ethtool private flag to make forcing link down optional

Add new infrastructure for implementing ethtool private flags using the
existing pf->flags bitmap to store them, and add the link-down-on-close
ethtool private flag to optionally bring down the PHY link when the
interface is administratively downed.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>