mtd: onenand: remove redundant variable ooblen

Variable ooblen is being initialized with a value that is never read.
The variable is never used after this, so it is redundant and can be
removed.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20211205230729.79337-1-colin.i.king@gmail.com