igb: Add check for invalid size to igb_get_invariants_82575()

Recent commits have changed how EEPROM size is checked and if the size
word is misconfigured, the driver will fail to load.  This patch adds a
check for invalid size word in the EEPROM and uses default size instead
for 82576 parts.

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: Jeff Kirsher <jeffrey.t.kirsher@intel.com>