mmc: sdhci: Check mrq != NULL in sdhci_tasklet_finish

It seems that under certain circumstances the sdhci_tasklet_finish()
call can be entered with mrq set to NULL, causing the system to crash
with a NULL pointer de-reference.

Seen on S3C6410 system.  Based on a patch by Dimitris Papastamos.
Reported-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Cc: <stable@kernel.org>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/host/sdhci.c

@@ -1334,6 +1334,13 @@ static void sdhci_tasklet_finish(unsigned long param)
 
 	host = (struct sdhci_host*)param;
 
+        /*
+         * If this tasklet gets rescheduled while running, it will
+         * be run again afterwards but without any active request.
+         */
+	if (!host->mrq)
+		return;
+
 	spin_lock_irqsave(&host->lock, flags);
 
 	del_timer(&host->timer);