With spurious interrupt cmd can be null even when we have CC
set in irq status.

Fixes: NB#106295 - prevent potential kernel crash in the MMC driver
Signed-off-by: Jarkko Lavinen <jarkko.lavinen@nokia.com>
Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Pierre Ossman <pierre@ossman.eu>