rt2x00: Don't treat ATIM queue as second beacon queue.

Current code for the atim queue is strange, as it is considered in the
rt2x00_dev structure as a second beacon queue.
Normalize this by letting the atim queue have its own struct data_queue
pointer in the rt2x00_dev structure.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>