mtd: rawnand: brcmnand: Fix BCH ECC layout for large page NAND parts

The way oobregion->offset is derived for large page NAND parts is
wrong, fixes it.

Fixes: ef5eeea6 ("mtd: nand: brcm: switch to mtd_ooblayout_ops")
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>