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>

drivers/mmc/host/dw_mmc.c

@@ -2217,6 +2217,10 @@ static irqreturn_t dw_mci_interrupt(int irq, void *dev_id)
 		/* Handle SDIO Interrupts */
 		for (i = 0; i < host->num_slots; i++) {
 			struct dw_mci_slot *slot = host->slot[i];
+
+			if (!slot)
+				continue;
+
 			if (pending & SDMMC_INT_SDIO(slot->sdio_id)) {
 				mci_writel(host, RINTSTS,
 					   SDMMC_INT_SDIO(slot->sdio_id));