mtd: onenand: use mtd_ooblayout_xxx() helpers where appropriate

The mtd_ooblayout_xxx() helper functions have been added to avoid direct
accesses to the ecclayout field, and thus ease for future reworks.
Use these helpers in all places where the oobfree[] and eccpos[] arrays
where directly accessed.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>