iwlwifi: move _agn statistics related structure

agn and 3945 has different statistics_notif data structure; since 3945
has it statistics_notif data structure inside the _3945 portion of
iwl_priv, it make sense to move the agn statistics_notif into _agn
portion.

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