ixgbe: Fix flow control for Xeon D KR backplane

Xeon D KR backplane is different from other backplanes,
in that we can't use auto-negotiation to determine the
mode. Instead, use whatever the user configured.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>