rt2x00: rt2800usb: fix status register reread logic

Another good catch from Jakub Kicinski. This patch fixes my
recent commit: ed61e2b
"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>

drivers/net/wireless/rt2x00/rt2800usb.c

@@ -184,7 +184,7 @@ static bool rt2800usb_tx_sta_fifo_read_completed(struct rt2x00_dev *rt2x00dev,
 	 * 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;