rt2x00: Minor optimizazion in txdone path

We can save an indirect function call + some instructions for fetching
the actual function pointer by passing the driver specific txwi pointer
directly from rt2800pci/rt2800usb to rt2800lib instead of using the
rt2800_drv_get_txwi callback.

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