Another good catch from Jakub Kicinski. This patch fixes my
recent commit: 
ed61e2b02027935520d1be884fac0b2ffce8379a
"rt2x00: rt2800usb: rework txdone code"
We should reread status register only when nobody else start already
reading status i.e. test_and_set_bit(TX_STATUS_READING, flags) return 0.
Reported-by: Jakub Kicinski <moorray@wp.pl>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
         * here again if status reading is needed.
         */
        if (rt2800usb_txstatus_pending(rt2x00dev) &&
-           test_and_set_bit(TX_STATUS_READING, &rt2x00dev->flags))
+           !test_and_set_bit(TX_STATUS_READING, &rt2x00dev->flags))
                return true;
        else
                return false;