igb: Update license text to remove FSF address and update copyright.

This patch updates the license text to remove address of Free Software
Foundation and refer  users to www.gnu.org instead. This patch also updates
the copyright dates in appropriate igb driver files.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>