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

See commit: c0cbfd0e
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 <ralf.roesch@rw-gmbh.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Acked-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>