drivers/mmc/mmc.c: Fix build warning

Fix:
mmc.c: In function 'mmc_bounce_buffer_start':
mmc.c:132:13: warning: no return statement in function returning
non-void [-Wreturn-type]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Andy Fleming <afleming@gmail.com>
Cc: Marek Vasut <marex@denx.de>
Acked-by: Marek Vasut <marex@denx.de>

drivers/mmc/mmc.c

@@ -129,7 +129,7 @@ static void mmc_bounce_buffer_stop(struct mmc_data *backup,
 }
 #else
 static inline int mmc_bounce_buffer_start(struct mmc_data *backup,
-					struct mmc_data *orig) { }
+					struct mmc_data *orig) { return 0; }
 static inline void mmc_bounce_buffer_stop(struct mmc_data *backup,
 					struct mmc_data *orig) { }
 #endif