brcmfmac: fix a double spin_unlock_irqrestore issue in dpc

dpc_tl_lock is not acquired in the error handle code for bus down.
But it's unlocked using spin_unlock_irqrestore after finishing task
list walk down. Grab the lock before breaking the loop to avoid a
double unlock.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NFranky Lin <frankyl@broadcom.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

drivers/net/wireless/brcm80211/brcmfmac/dhd_sdio.c

@@ -2637,6 +2637,7 @@ static int brcmf_sdbrcm_dpc_thread(void *data)
 				/* after stopping the bus, exit thread */
 				brcmf_sdbrcm_bus_stop(bus->sdiodev->dev);
 				bus->dpc_tsk = NULL;
+				spin_lock_irqsave(&bus->dpc_tl_lock, flags);
 				break;
 			}