[MTD] [OneNAND] Fix access the past of the real oobfree array

Here it's not the case: all the entries are occupied by
OOB chunks. Therefore, once we get into a loop like

        for (free = this->ecclayout->oobfree; free->length; ++free) {
	}

we might end up scanning past the real oobfree array.

Probably the best way out, as the same thing might happen for common NAND
as well, is to check index against MTD_MAX_OOBFREE_ENTRIES.

Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>