mtd: nand_base: do not cache pages with uncorrectable ECC errors

Currently MTD caches the last read NAND page, even if there was an uncorrectable ECC
error. This patch prevents caching in case of uncorrectable ECC errors. The reason
is that we want to allow the user to re-read the NAND page several times. In case of
unstable bits re-trying may help.

Moreover, current behavior is wrong because the first read returns -EBADMSG (correctly)
but the second read succeeds and incorrectly returns 0 (because we read from the cache).

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>