e1000e: 82571-based mezzanine card can fail ethtool link test

On certain 82571-based mezzanine NICs in some blade servers, the ethtool
link test can fail due to the serdes_has_link flag not set correctly.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Emil Tantilov <emil.s.tantilov@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>