e1000e: store EEPROM version number to prevent unnecessary NVM reads

Rather than reading the NVM to get the EEPROM version number everytime the
ethool get_drvinfo function is called, read it once during probe and save
it for future reference.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>