mtd: nand: atmel: Fix get_sectorsize() function

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: f88fc122cc34 ("mtd: nand: Cleanup/rework the atmel_nand driver")
Cc: <[email protected]>
Reported-by: Olivier Schonken <[email protected]>
Signed-off-by: Boris Brezillon <[email protected]>
Reviewed-by: Richard Weinberger <[email protected]>
Acked-by: Nicolas Ferre <[email protected]>
Tested-by: Olivier Schonken <[email protected]>