mmc: sh_mmcif: Check for null res pointer

If there is no suitable resource, platform_get_resource() will return
NULL.
Therefore in order to avoid the dereference of the NULL pointer, it
should be better to check the 'res'.

Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Cc: stable@vger.kernel.org # v5.16+
Link: https://lore.kernel.org/r/20220119120006.1426964-1-jiasheng@iscas.ac.cn


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>