iwlwifi: fix EEPROM enhance tx power offset

Set the correct EEPROM offset for enhance tx power for 6000 series

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>