mmc: fix rmmod race for hosts using card-detection polling

MMC hosts that poll for card detection by defining the MMC_CAP_NEEDS_POLL
flag have a race on rmmod, where the delayed work is cancelled without
waiting for completed polling. To prevent this a _sync version of the work
cancellation has to be used.
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/core/core.c

@@ -1559,7 +1559,7 @@ void mmc_stop_host(struct mmc_host *host)
 
 	if (host->caps & MMC_CAP_DISABLE)
 		cancel_delayed_work(&host->disable);
-	cancel_delayed_work(&host->detect);
+	cancel_delayed_work_sync(&host->detect);
 	mmc_flush_scheduled_work();
 
 	/* clear pm flags now and let card drivers set them as needed */