mmc: core: Fixup Oops for SDIO shutdown

Commit "mmc: core: Handle card shutdown from mmc_bus" introduced an
Oops in the shutdown sequence for SDIO.

The drv pointer, does not exist for SDIO since the probing of the SDIO
card from the mmc_bus perspective is expected to fail by returning
-ENODEV.

This patch adds the proper check for the pointer before calling it.
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reported-by: Stephen Warren <swarren@wwwdotorg.org>
Reported-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Tested-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>

drivers/mmc/core/bus.c

@@ -129,7 +129,8 @@ static void mmc_bus_shutdown(struct device *dev)
 	struct mmc_host *host = card->host;
 	int ret;
 
-	drv->shutdown(card);
+	if (dev->driver && drv->shutdown)
+		drv->shutdown(card);
 
 	if (host->bus_ops->shutdown) {
 		ret = host->bus_ops->shutdown(host);