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 <[e-mail]>
Reported-by: Stephen Warren <[e-mail]>
Reported-by: Tuomas Tynkkynen <[e-mail]>
Tested-by: Tuomas Tynkkynen <[e-mail]>
Acked-by: Jaehoon Chung <[e-mail]>
Signed-off-by: Chris Ball <[e-mail]>

drivers/mmc/core/bus.c

···
 	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);