mtd: txx9ndfmc: limit transfer bytes to 512 (ECC provides 6 bytes max)

See commit: c0cbfd0e81d879a950ba6f0df3f75ea30c5ab16e
Using __nand_correct_data() helper function, this driver can read 512
byte (with 6 byte ECC) at a time.

This is correct, but not more:
With NAND chips providing page sizes > 512 Bytes
chip->ecc.bytes are calculated > 6 in txx9ndfmc_nand_scan.
According the data sheet there are (only) 6 bytes ECC available.

After applying the patch a Hynix 512M*8 with 2KiB page size could be
successfully formatted and used with an ubifs file system.

Signed-off-by: Ralf Roesch <[e-mail]>
Signed-off-by: Artem Bityutskiy <[e-mail]>
Acked-by: Atsushi Nemoto <[e-mail]>
Signed-off-by: David Woodhouse <[e-mail]>

drivers/mtd/nand/txx9ndfmc.c

···
 	ret = nand_scan_ident(mtd, 1, NULL);
 	if (!ret) {
 		if (mtd->writesize >= 512) {
-			chip->ecc.size = mtd->writesize;
-			chip->ecc.bytes = 3 * (mtd->writesize / 256);
+			/* Hardware ECC 6 byte ECC per 512 Byte data */
+			chip->ecc.size = 512;
+			chip->ecc.bytes = 6;
 		}
 		ret = nand_scan_tail(mtd);
 	}