iwlwifi: move agn common code to iwlagn library file

Multiple iwlagn based devices share the same common functions.
Move those functions from iwl-5000.c to iwl-agn-lib.c file.

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