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.h

@@ -21,7 +21,10 @@ struct capwap_read_message_elements_ops {
 	int (*read_block)(capwap_message_elements_handle handle, uint8_t* data, unsigned short length);
 };
 
-struct capwap_message_elements_ops {
+struct capwap_message_elements_ops
+{
+	int category;
+
 	/* Build message element */
 	void (*create)(void* data, capwap_message_elements_handle handle, struct capwap_write_message_elements_ops* func);