ixgbe: Properly display 1 gig downshift warning for backplane

Description: When using Intel smartspeed, the patch displays a
warning when the link down shifts to 1 Gig.

Signed-off-by: Anjali Singhai <anjali.singhai@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>