mtdchar: fix usage of mtd_ooblayout_ecc()

Section was not properly computed. The value of OOB region definition is
always ECC section 0 information in the OOB area, but we want to get all
the ECC bytes information, so we should call
mtd_ooblayout_ecc(mtd, section++, &oobregion) until it returns -ERANGE.

Fixes: c2b78452 ("mtd: use mtd_ooblayout_xxx() helpers where appropriate")
Cc: <stable@vger.kernel.org>
Signed-off-by: OuYang ZhiZhong <ouyzz@yealink.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>