mtd: nand: marvell: remove redundant variable 'oob_len'

Variable oob_len is assigned and never read, hence it is redundant and
can be removed.

Cleans up clang warnings:

drivers/mtd/nand/marvell_nand.c:1356:6: warning: Value stored to 'oob_len'
during its initialization is never read
drivers/mtd/nand/marvell_nand.c:1369:4: warning: Value stored to 'oob_len'
is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>