rt2x00: Limit TX done looping to number of TX ring entries

Similar to rt2800pci, remove the check for duplicate
register reading, and instead limit the for-loop to
the maximum number of TX entries inside a queue.

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