[MTD] [OneNAND] proper onenand_bbt_read_oob() prototype

This patch adds a proper prototype for onenand_bbt_read_oob() in
include/linux/mtd/onenand.h

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>