iwlwifi: move agn only eeprom functions to separate file

Some of the functions in iwl-eeprom.c file are for agn devices only,
Those functions do not have to be part of iwlcore.ko, so move those
to iwl-agn-eeprom.c file.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>