rt2x00: Reduce tx descriptor size

The tx descriptor values qid, cw_min, cw_max and aifs are directly
accessible through the tx entry struct. So there's no need to copy
them into the tx descriptor and passing them to the indiviual drivers.
Instead we can just get the correct value from the tx entry.

Signed-off-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>