igb: Fix for failure to init on some 82576 devices.

Move nvm invalid size check to before size assigned by mac_type for
82575 and later parts in get_invariants function.  This fixes a problem
found on some 82576 devices where the part will not initialize because
the nvm_read function pointer ends up getting assigned to the incorrect
function.

Reported By: Stefan Assmann <sassmann@redhat.com>
Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Tested-by: Jeff Pieper <jeffrey.e.pieper@intel.com>
Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>