mtd: nand: atmel: Fix get_sectorsize() function

get_sectorsize() was not using the appropriate macro to extract the
ECC sector size from the config cache, which led to buggy ECC when
using 1024 byte sectors.

Fixes: f88fc122

 ("mtd: nand: Cleanup/rework the atmel_nand driver")
Cc: <stable@vger.kernel.org>
Reported-by: Olivier Schonken <olivier.schonken@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Tested-by: Olivier Schonken <olivier.schonken@gmail.com>