Pausing queue on flush make no sense since txdone procedure un-pause
queue. Before flush procedure we have to assure queue is stopped,
i.e. on receive path h/w RX is disabled, on transmit path queue is
disabled in mac80211. That conditions are true except one function:
rt2x00usb_watchdog_tx_dma(), so add stop/start queue there.

Note stop/start queue can be racy if we do this from multiple paths,
but currently we stop TX queues only on rt2x00lib_disable_radio(),
which also stop/sync watchdog, hance we have no race condition.
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>