iwlwifi: identify eeprom version for 6x50 series NIC

Adding support for 6x50 series EEPROM version check, 6x50 is wifi/wimax
combo device which has different EEPROM map

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>