mtd: mtdchar: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of letting the code fall
through to the next case.

Link: https://github.com/KSPP/linux/issues/115Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210305082224.GA137360@embeddedor

drivers/mtd/mtdchar.c

@@ -983,6 +983,7 @@ static int mtdchar_ioctl(struct file *file, u_int cmd, u_long arg)
 			if (!mtd_has_oob(mtd))
 				return -EOPNOTSUPP;
 			mfi->mode = arg;
+			break;
 
 		case MTD_FILE_MODE_NORMAL:
 			break;