mmc: sdhci: Check mrq != NULL in sdhci_tasklet_finish

mmc: sdhci: Check mrq != NULL in sdhci_tasklet_finish

commit 0c9c99a765321104cc5f9c97f949382a9ba4927e upstream.

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>
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>