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>