iwlwifi: code cleanup for _agn devices

Move configuration structure for _agn devices from iwl-dev.h to
iwl-agn.h. Those data structures are for _agn devices and should be keep
for _agn devices only.

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