iwlwifi: move hcmd related code to separate file

Multiple iwlagn based devices shared the same hcmd related functions.
Move from iwl-5000.c to iwl-agn-hcmd.c file.

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