iwlwifi: define structures and functions externally for customization

defined the structures and functions as extern to alter behavior used by
5000 series for other products including 100 and 6000 series

Signed-off-by: Jay Sternberg <jay.e.sternberg@linux.intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>