UBI: use mtd->writebufsize to set minimal I/O unit size

Previously we used mtd->writesize field to set UBI's minimal
I/O unit size. This sometimes caused UBIFS recovery issues
when mounting an uncleanly unmounted UBIFS partition on NOR
flash since mtd->writesize is 1 byte for NOR flash. The
MTD CFI driver however often performs writing multiple
bytes in one programming operation using the chip's write
buffer. We have to use the size of this write buffer as
a minimal I/O unit size for UBI on NOR flash to fix the
observed UBIFS recovery issues.

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>