rt2x00: Fix crash on USB unplug

rt2x00: Fix crash on USB unplug

commit 070192dd2975c0e97bbdeac7623b755235c6db7d upstream.

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>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>