warning: comparison of unsigned expression < 0 is always false

Remove an unnecessary test that is reported when compiling driver with W=1.
The test is unnecessary because Intel wired GbE hardware older (i.e. less)
than 82571 is not supported by this driver.
Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>