[MTD] [NAND] nand_ecc.c: fix big endian, strengthen test, add printk

This patch for nand_ecc.c fixes three issues

- fix code so it also works on big endian architectures
- added a printk in case of an uncorrectable ecc error
- strengthen the test for correctable errors (decreasing the chance
  that multiple bit faults by accident will be seen as correctable)

Note: the big endian code is only tested in a testbed (running on big endian
hardware) as I cannot rebuild and test a big endian kernel at the moment.
However the only thing that can go wrong is if <asm/byteorder.h> does not
give __BIG_ENDIAN in that case. In my eyes very unlikely.

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