i40e: Fix for ethtool Supported link modes

This patch fixes the problem where the ethtool Supported link
modes list backplane interfaces on X722 devices for 10GbE with
SFP+ and Cortina retimer. This patch fixes the problem by setting
and using a flag for this particular device since the backplane
interface is only between the internal PHY and the retimer and it
should not be seen by the user as they cannot use it.
Without this patch, the user wrongly thinks that backplane interfaces
are supported on their device when they actually are not.

Change-ID: I3882bc2928431d48a2db03a51a713a1f681a79e9
Signed-off-by: Harshitha Ramamurthy <harshitha.ramamurthy@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>