igb: Add new function to read part number from EEPROM in string format

New adapters will have part numbers stored in string format rather than
simple hex format. This function will read part number formats in either
hex or string.

Signed-off-by: Carolyn Wyborny <carolyn.wyborny@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>