NAND: Update nand_ecc.c to latest Linux version

This patch updates the nand_ecc code to the latest Linux version.
The main reason for this is the more compact code. This makes
it possible to include the ECC code into the NAND bootloader
image (NAND_SPL) for PPC4xx.

Signed-off-by: Stefan Roese <sr@denx.de>