mmc: toshsd: move dereference below check for NULL

We check for NULL pointers after dereferencing so it's too late.  Oddly
enough, Smatch misses this code but complains about the caller passing
NULL pointers to this function:

	drivers/mmc/host/toshsd.c:389 toshsd_irq()
	error: we previously assumed 'host->cmd' could be null (see line 349)

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>