Fix comment in include/linux/mmc/host.h

In include/linux/mmc/host.h, it is mentionned that the callback to
know if a card is present or not is get_ro(). But it's get_cd().
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>

include/linux/mmc/host.h

@@ -65,7 +65,7 @@ struct mmc_host_ops {
 	 *   -ENOSYS when not supported (equal to NULL callback)
 	 *   or a negative errno value when something bad happened
 	 *
-	 * Return values for the get_ro callback should be:
+	 * Return values for the get_cd callback should be:
 	 *   0 for a absent card
 	 *   1 for a present card
 	 *   -ENOSYS when not supported (equal to NULL callback)