[MTD] [NAND] nand_ecc.c: rewrite for improved performance

This patch improves the performance of the ecc generation code by a
factor of 18 on an INTEL D920 CPU, a factor of 7 on MIPS and a factor of 5
on ARM (NSLU2)

Signed-off-by: Frans Meulenbroeks <fransmeulenbroeks@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>