rt2x00: Store queue idx and entry idx in data_ring and data_entry

Store the queue idx inside structure data_ring
Store the entry idx inside structure data_entry
This saves us a few calls to ARRAY_INDEX() which is now unused.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>