igb: add check for fiber/serdes devices to igb_set_spd_dplx;

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>