mmc: replace host->card_busy

As card_busy was only used to indicate if the host was exclusively
claimed and not really used to identify a particular card, replacing
it with just a boolean makes things a lot more easily understandable.

Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>
diff --git a/include/linux/mmc/host.h b/include/linux/mmc/host.h
index c15ae19..dc4c6e3 100644
--- a/include/linux/mmc/host.h
+++ b/include/linux/mmc/host.h
@@ -106,8 +106,9 @@
 	struct list_head	cards;		/* devices attached to this host */
 
 	wait_queue_head_t	wq;
-	spinlock_t		lock;		/* card_busy lock */
-	struct mmc_card		*card_busy;	/* the MMC card claiming host */
+	spinlock_t		lock;		/* claimed lock */
+	unsigned int		claimed:1;	/* host exclusively claimed */
+
 	struct mmc_card		*card_selected;	/* the selected MMC card */
 
 	struct delayed_work	detect;