igb: Update link modes display in ethtool

This patch fixes multiple problems in the link modes display in ethtool.
Newer parts have more complicated methods to determine actual link
capabilities.  Older parts cannot communicate with their SFP modules.
Finally, all the available defines are not displayed by ethtool.  This
updates the link modes to be as accurate as possible depending on what data
is available to the driver at any given time.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>