mmc: dw_mmc: Fix NULL pointer dereference

If mrq->sbc is not NULL but data->stop happens to be NULL,
it will lead to NULL pointer dereferencing. Avoid this by
having a NULL check for data->stop.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>