rt2x00: Fix crash on USB unplug

By not scheduling the TX/RX completion worker threads
when Radio is disabled, or hardware has been unplugged,
the queues cannot be completely cleaned.

This causes crashes when the hardware has been unplugged while
the radio is still enabled.

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>