UBI: fix compile warning

cdev.c whines in current git:

drivers/mtd/ubi/cdev.c: In function `major_to_device':
drivers/mtd/ubi/cdev.c:67: warning: control reaches end of non-void function

Shut it up.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>

drivers/mtd/ubi/cdev.c

@@ -64,6 +64,7 @@ static struct ubi_device *major_to_device(int major)
 		if (ubi_devices[i] && ubi_devices[i]->major == major)
 			return ubi_devices[i];
 	BUG();
+	return NULL;
 }
 
 /**