iwlwifi: code cleanup for generic defines

Some defines used by all agn devices, but the definitions were in
iwl-4965-hw.h, move those to iwl-agn-hw.h which is the better place for
those.

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