mmc: dw_mmc: Don't crash if we get an interrupt before slot has initted

It's unlikely that this is really needed on any single-slot systems
where we disable card detects until the end of probe, but it still
seems safer to check to make sure that a slot has been initted before
we try to dereference it to find the SDIO interrupt mask.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>