iwlwifi: move ucode loading related code to separated file

Multiple iwlagn based devices shared the same ucode loading procedure.
Move loading related code from iwl-5000.c to iwl-agn-ucode.c file.

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