move the category setting into the element ops

this prepares the element ops for decoding vendor
ies into seperate types.

It also makes sense to keep all information about a IE
in one place.

src/common/capwap_element_addstation.c

@@ -127,6 +127,7 @@ static void* capwap_addstation_element_parsing(capwap_message_elements_handle ha
 
 /* */
 struct capwap_message_elements_ops capwap_element_addstation_ops = {
+	.category = CAPWAP_MESSAGE_ELEMENT_SINGLE,
 	.create = capwap_addstation_element_create,
 	.parse = capwap_addstation_element_parsing,
 	.clone = capwap_addstation_element_clone,