iwlwifi: code cleanup for "load ucode" function

Loading uCode functions are shared across multiple agn devices.
Move those functions to iwl-agn-ucode.c file with other uCode
related functions.

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