mtd: sh_flctl: Restructure the hardware ECC handling

There are multiple reasons for a rewrite:
 - a race exists: when _4ECCEND is set, _4ECCFA may become true too
   meanwhile, which is lost and a non-correctable error is treated as
   correctable.
 - the ECC statistics don't get properly propagated to the base code.
 - empty pages would get marked as corrupted

The rewrite resolves the issues and I hope it gives a more explicit
code flow structure.

Signed-off-by: Bastian Hecht <hechtb@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>