mmc: dw_mmc: Fix potential null pointer risk

we previously assumed 'host->slot' could be null,
null pointer judgment should be added

Signed-off-by: Wen Zhiwei <wenzhiwei@kylinos.cn>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Link: https://lore.kernel.org/r/20211229023814.53372-1-wenzhiwei@kylinos.cn
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>