[MTD] [NOR] STAA: use writesize instead off eccsize to represent ECC block

The cfi_staa_write_buffers() uses mtd->eccsize but means mtd->writesize.
BTW, mtd-eccsize is broken and is not initialized, which means the code
fixed by this patch is broken/unused anyway.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>