i40e: add switch for link polling

There's been some need for controlling the periodic link polling for
debugging link issues.  This patch enables switching it off and on
through an ethtool private flag.  The link poll remains on by default,
but can be turned off with
	ethtool --set-priv-flags p261p1 LinkPolling off
and later turned back on with
	ethtool --set-priv-flags p261p1 LinkPolling on

To check the current status, use
	ethtool --show-priv-flags p261p1

Change-ID: I32e4ab654ff3eec90a06cf144899971b82d71c40
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>