rt2x00: Remove failsave from rt2x00usb_watchdog_tx_dma

When the TX status handler failed to clear the queue
in rt2x00usb_watchdog_tx_dma() we shouldn't use a failsave
to use the rt2x00usb txdone handler.

If a driver has overriden the txdone handler it must make
sure the txdone handler is capable of cleaning up the queue itself.

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>