mtd: nand: fix tmio_nand ecc correction

This driver may be reading 512 bytes at a times, but still calculates
256-byte sector ECC.  So the nand_correct_data() is not appropriate
for this driver.  Implement its ecc.correct function calling
__nand_correct_data() twice.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Vimal Singh <vimalsingh@ti.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>