rt2x00: Correctly initialize data and desc pointer

rt2500usb and rt73usb store the descriptor in different
places. This means we should move the initialization of
the 2 pointers to the driver callback function fill_rxdone().

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