iwlwifi: move hw related defines to separate file

Multiple iwlagn based devices shared the same hw definitions.
Move device hardware related defines from iwl-5000-hw.h to iwl-agn-hw.h
file.

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