rt2x00: Remove TX_MGMT queue usage

rt73usb doesn't have a 5th queue, correctly initialize hw->queues
rt61pci had a 5th queue but was removed recently, correctly initialize
hw->queues and remove the last remnants.

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