iwlwifi: generic parameter define for _agn device

Code clean up to change name from having 5000 as part of name which easy
to confuse and think it is for 5000 series devices to more
generic _agn name since it is being used by multiple _agn devices.

No functional changes.

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